It is not a good idea to use Strings with Arduino. They cause memory problems.
Use character arrays instead, and post your code properly, so it looks like this similar program using character arrays;
char message[21]; //20 characters plus zero terminator
int x=0; //array index
void setup() {
// Open serial communications and wait for port to open:
Serial.begin(9600);
while (!Serial) {
; // wait for serial port to connect
}
// send an intro:
Serial.println("Code Initialized");
}
void loop() {
if (Serial.available() > 0) {
message[x] = Serial.read(); // add incoming characters to the array
x++; //increment index
message[x]=0; //terminate character string
}
if ( strlen(message) == 20 ) { //if 20 characters have been read and stored, print the string
Serial.println(message);
x=0; //start over from beginning
message[x]=0; //empty message
}
}