What I am saying is that if the condition is not met only that one line will be missed. I would have thought if you can not open the file you do not want to read from it.
if (Serial.available() > 0)
{
incomingByte = Serial.read();
if (incomingByte == 'R')
{
Serial.begin(115200);
if (!myFile.open("codes.txt", O_READ))
Serial.println("code.txt:"); // miss this if the file is not open and do the rest of the lines
int data;
while ((data = myFile.read()) > 0) Serial.write(data);
// close the file:
myFile.close();
}