MY Code doesn´t work can anybody help me please???

unsigned long Start1; //Läufer1
unsigned long Stopp1;

unsigned long Start2; //Läufer2
unsigned long Stopp2;

unsigned long Start3; //Läufer3
unsigned long Stopp3;

unsigned long Start4; //Läufer4
unsigned long Stopp4;

unsigned long Start5; //Läufer5
unsigned long Stopp5;

unsigned long Start6; //Läufer6
unsigned long Stopp6;

unsigned long Start7; //Läufer7
unsigned long Stopp7;

unsigned long Start8; //Läufer8
unsigned long Stopp8;

unsigned long Start9; //Läufer9
unsigned long Stopp9;

unsigned long Start10; //Läufer10
unsigned long Stopp10;

const byte interruptPin2 = 2; //START
const byte interruptPin3 = 3; //STOPP

int nachsterLauferStart;
int nachsterLauferStopp;

unsigned long Zeit1;
unsigned long Zeit2;
unsigned long Zeit3;
unsigned long Zeit4;
unsigned long Zeit5;
unsigned long Zeit6;
unsigned long Zeit7;
unsigned long Zeit8;
unsigned long Zeit9;
unsigned long Zeit10;

void setup()
{
pinMode(interruptPin2, INPUT_PULLUP);
pinMode(interruptPin3, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(interruptPin2), Start, LOW);
attachInterrupt(digitalPinToInterrupt(interruptPin3), Stopp, LOW);
Serial.begin(9600);

}

void Start()
{
if (nachsterLauferStart==1)
{
Start1 = millis();

}
if (nachsterLauferStart==2)
{
Start2 = millis();

}
if (nachsterLauferStart==3)
{
Start3 = millis();

}
if (nachsterLauferStart==4)
{
Start4 = millis();

}
if (nachsterLauferStart==5)
{
Start5 = millis();

}
if (nachsterLauferStart==6)
{
Start6 = millis();

}
if (nachsterLauferStart==7)
{
Start7 = millis();

}
if (nachsterLauferStart==8)
{
Start8 = millis();

}
if (nachsterLauferStart==9)
{
Start9 = millis();

}
if (nachsterLauferStart==10)
{
Start10 = millis();

}
nachsterLauferStart++;
}

void Stopp()
{
if (nachsterLauferStopp==1)
{
Stopp1 = millis();
Stopp1-=Start1 == Zeit1;
Serial.println("Laufer1" + Zeit1);
}
if (nachsterLauferStopp==2)
{
Stopp2 = millis();
Stopp2-=Start2 == Zeit2;
Serial.println("Laufer2" + Zeit2);

}
if (nachsterLauferStopp==3)
{
Stopp3 = millis();
Stopp3-=Start3 == Zeit3;
Serial.println("Laufer3" + Zeit3);

}
if (nachsterLauferStopp==4)
{
Stopp4 = millis();
Stopp4-=Start4 == Zeit4;
Serial.println("Laufer4" + Zeit4);

}
if (nachsterLauferStopp==5)
{
Stopp5 = millis();
Stopp5-=Start5 == Zeit5;
Serial.println("Laufer5" + Zeit5);

}
if (nachsterLauferStopp==6)
{
Stopp6 = millis();
Stopp6-=Start6 == Zeit6;
Serial.println("Laufer6" + Zeit6);

}
if (nachsterLauferStopp==7)
{
Stopp7 = millis();
Stopp7-=Start7 == Zeit7;
Serial.println("Laufer7" + Zeit7);

}
if (nachsterLauferStopp==8)
{
Stopp8 = millis();
Stopp8-=Start8 == Zeit8;
Serial.println("Laufer8" + Zeit8);

}
if (nachsterLauferStopp==9)
{
Stopp9 = millis();
Stopp9-=Start9 == Zeit9;
Serial.println("Laufer9" + Zeit9);

}
if (nachsterLauferStopp==10)
{
Stopp10 = millis();
Stopp10-=Start10 == Zeit10;
Serial.println("Laufer10" + Zeit10);

}

nachsterLauferStopp++;
}

You want the code to do something but you have not told us what that is or how the behavior of this code differs from your desire.

Furthermore, when you are tempted to use numbers when naming your variables, it is time to learn about arrays.

It's also time to learn how to use Ctrl+t in the IDE to format your code, and to use code tags when posting your code.

Where is your loop()?

What is this supposed to do?

Stopp1 -= Start1 == Zeit1;

You forgot to have a loop() function.

Here is how you would do it with arrays:

const byte SampleCount = 10;

unsigned long Start[SampleCount];        //Läufer1
unsigned long Stopp[SampleCount];
unsigned long Zeit[SampleCount];

const byte interruptPin2 = 2;     //START
const byte interruptPin3 = 3;     //STOPP

int nachsterLauferStart;
int nachsterLauferStopp;

void setup()
{
  pinMode(interruptPin2, INPUT_PULLUP);
  pinMode(interruptPin3, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(interruptPin2), StartISR, LOW);
  attachInterrupt(digitalPinToInterrupt(interruptPin3), StoppISR, LOW);
  Serial.begin(9600);
}

void StartISR()
{
  if (nachsterLauferStart < SampleCount)
  {
    Start[nachsterLauferStart] = millis();
    nachsterLauferStart++;
  }
}

void StoppISR()
{
  if (nachsterLauferStopp < SampleCount)
  {
    Stopp[nachsterLauferStopp] = millis();

    Stopp[nachsterLauferStopp] -= (Start[nachsterLauferStopp] == Zeit[nachsterLauferStopp]);  // ??????
    // Did you mean:
    // Zeit[nachsterLauferStopp] = Stopp[nachsterLauferStopp] - Start[nachsterLauferStopp];

    // DON'T DO Serial.print INSIDE AN ISR!
    // Serial.print("Laufer");
    // Serial.print(nachsterLauferStopp);
    // Serial.print(" = ");
    // Serial.print(Zeit[nachsterLauferStopp]);

    nachsterLauferStopp++;
  }
}

To move the printing out of the ISR (where it isn't very safe), try:

void StoppISR()
{
  if (nachsterLauferStopp < SampleCount)
  {
    Stopp[nachsterLauferStopp] = millis();
    nachsterLauferStopp++;
  }
}

void loop()
{
  static byte index = 0;

  if (index < nachsterLauferStopp)
  {
    unsigned long zeit = Stopp[index] - Start[index];

    Serial.print("Laufer");
    Serial.print(index);
    Serial.print(" = ");
    Serial.print(zeit);

    index++;
  }
}