If the the file cant be open then skip everything else.
In which case that code should be:-
if (Serial.available() > 0)
{
incomingByte = Serial.read();
if (incomingByte == 'R')
{
Serial.begin(115200);
if (!myFile.open("codes.txt", O_READ)) { // you missed this opening brace
Serial.println("code.txt:");
int data;
while ((data = myFile.read()) > 0) Serial.write(data);
// close the file:
myFile.close();
}
} // this is the matching closing brace
}