Storage size of 'array' isn't known

This project was created because of an earlier one, a magnetic self-balancing platform used to demonstrate the principles of Maglev control. I then wanted to add a Kalman filter, which uses both accelerator and gyroscope data at once to correct the imperfections in the gyroscope, which would otherwise result in drift, especially over long periods of time.

An easier option is to add calibration that assumes the gyroscope is not moving; however, it doesn't work as well. When I have runned the code it is showing below error. I don't know how to solve the problem kindly help me to solve the problem.

#include<Wire.h>

#define gyro_address 0x68 

int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

array gyro_data = []; //this stores the past data from the sensor, giving the kalman filter some of its neccasary data.
array accel_data = []; //this stores the past data from the sensor, giving the kalman filter some of its neccasary data.
array gyro_current = []; //this feeds into the main variable, but it's here just to better orginize the X, Y. & Z values.
array accel_current = []; //this feeds into the main variable, but it's here just to better orginize the X, Y. & Z values.

void setup() {
  Wire.begin();
  Wire.beginTransmission(gyro_address);
  Wire.write(0x6B);  // PWR_MGMT_1 register
  Wire.write(0);     // set to zero (wakes up the MPU-6050)
  Wire.endTransmission(true); //set this to false, and it doesn't end. simple enough, right?
  
  Wire.beginTransmission(gyro_address);
  Wire.write(0x6a);
  Wire.requestFrom(gyro_address, 1, true);
  int usercontrol = Wire.read();
  Wire.endTransmission(true);
  Serial.begin(9600); 

  double x_rotation = 0, y_rotation = 0, z_rotation = 0; 
}

void loop() {
  Wire.beginTransmission(gyro_address);
  Wire.write(0x3B);  // starting with register 0x3B (ACCEL_XOUT_H).this number may change depending on how the gyroscope transferrs data.
  Wire.endTransmission(false);
  Wire.requestFrom(gyro_address, 14, true); // request a total of 14 registers (again, it may be different for your gyroscope)
  accel_x = Wire.read() << 8 | Wire.read(); // 0x3B (ACCEL_XOUT_H) & 0x3C (ACCEL_XOUT_L)
  accel_y = Wire.read() << 8 | Wire.read(); // 0x3D (ACCEL_YOUT_H) & 0x3E (ACCEL_YOUT_L)
  accel_z = Wire.read() << 8 | Wire.read(); // 0x3F (ACCEL_ZOUT_H) & 0x40 (ACCEL_ZOUT_L)
  temp = Wire.read() << 8 | Wire.read(); // 0x41 (TEMP_OUT_H) & 0x42 (TEMP_OUT_L)
  gyro_x = Wire.read() << 8 | Wire.read(); // 0x43 (GYRO_XOUT_H) & 0x44 (GYRO_XOUT_L)
  gyro_y = Wire.read() << 8 | Wire.read(); // 0x45 (GYRO_YOUT_H) & 0x46 (GYRO_YOUT_L)
  gyro_z = Wire.read() << 8 | Wire.read(); // 0x47 (GYRO_ZOUT_H) & 0x48 (GYRO_ZOUT_L)

   accel_current = [accel_x,accel_y,accel_z];
   gyro_current = [gyro_x,gyro_y,gyro_z];
   Serial.print(" | Accelerometer X:");
   Serial.print(accel_x);
   Serial.print(" | Accelerometer Y:");
   Serial.print(accel_y);
   Serial.print(" | Accelerometer Z:");
   Serial.print(accel_z);
   Serial.print(" | Temperature:");
   Serial.print(temp / 340.00 + 36.53);
   Serial.print(" | Gyro X:");
   Serial.print(gyro_x);
   Serial.print(" | Gyro Y:");
   Serial.print(gyro_y);
   Serial.print(" | Gyro Z:");
   Serial.println(gyro_z);
   gyro_data[gyro_data.length()] = gyro_current;
   accel_data[accel_data.length()] = accel_current;

   delay(100);//this is important to make sure the board and arduino both aren't overloading. slower-operating gyros may need a bigger number.
}

Error displayed

sketch_mar30b:26: error: storage size of 'array' isn't known

 int array [];

            ^

sketch_mar30b:28: error: 'array' does not name a type

 array gyro_data = []; //this stores the past data from the sensor, giving the kalman filter some of its neccasary data.

 ^

sketch_mar30b:29: error: 'array' does not name a type

 array accel_data = []; //this stores the past data from the sensor, giving the kalman filter some of its neccasary data.

 ^

sketch_mar30b:30: error: 'array' does not name a type

 array gyro_current = []; //this feeds into the main variable, but it's here just to better orginize the X, Y. & Z values.

 ^

sketch_mar30b:31: error: 'array' does not name a type

 array accel_current = []; //this feeds into the main variable, but it's here just to better orginize the X, Y. & Z values.

 ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino: In function 'void loop()':

sketch_mar30b:69: error: 'accel_current' was not declared in this scope

    accel_current = [accel_x,accel_y,accel_z];

    ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:9: note: 'int16_t accel_x' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

         ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:18: note: 'int16_t accel_y' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

                  ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:27: note: 'int16_t accel_z' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

                           ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino: In lambda function:

sketch_mar30b:69: error: expected '{' before ';' token

    accel_current = [accel_x,accel_y,accel_z];

                                             ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino: In function 'void loop()':

sketch_mar30b:70: error: 'gyro_current' was not declared in this scope

    gyro_current = [gyro_x,gyro_y,gyro_z];

    ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:42: note: 'int16_t gyro_x' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

                                          ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:50: note: 'int16_t gyro_y' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

                                                  ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino:19:58: note: 'int16_t gyro_z' declared here

 int16_t accel_x, accel_y, accel_z, temp, gyro_x, gyro_y, gyro_z; 

                                                          ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino: In lambda function:

sketch_mar30b:70: error: expected '{' before ';' token

    gyro_current = [gyro_x,gyro_y,gyro_z];

                                         ^

C:\Users\deb\AppData\Local\Temp\arduino_modified_sketch_610669\sketch_mar30b.ino: In function 'void loop()':

sketch_mar30b:85: error: 'gyro_data' was not declared in this scope

    gyro_data[gyro_data.length()] = gyro_current;

    ^

sketch_mar30b:86: error: 'accel_data' was not declared in this scope

    accel_data[accel_data.length()] = accel_current;

    ^

Multiple libraries were found for "Wire.h"
 Used: C:\Users\deb\AppData\Local\Arduino15\packages\Intel\hardware\i686\1.6.7+1.0\libraries\Wire
 Not used: C:\Users\deb\AppData\Local\Arduino15\packages\Intel\hardware\i686\1.6.7+1.0\libraries\Servo
 Not used: C:\Program Files (x86)\Arduino\libraries\Wire
Using library Wire at version 1.0 in folder: C:\Users\deb\AppData\Local\Arduino15\packages\Intel\hardware\i686\1.6.7+1.0\libraries\Wire 
exit status 1
storage size of 'array' isn't known[color=red][/color]
array gyro_data = [];

That is not how to declare an array in Arduino (C++). https://www.arduino.cc/en/Reference/Array

accel_current' was not declared in this scope

That is self explanatory.

I don't know exactly, can you give one sample from code. That will be easy to solve this issue. Arduino c++ new for me.

There are multiple code examples of how to declare an array at the link groundfungus shared with you. Please read it. After that read through the error messages and try your best to fix them all. Try compiling again and then if there are any errors try to fix those too. If you can't manage to fix some of the errors then come back here and post your revised code, error messages, and specific questions.