I seem to recall that newer board packages no longer implement serialEvent().
As you can see from the core file, the code just calls your serialEvent() function after loop(), so you can get the same result by calling serialEvent() as the last line in loop(), or just move the code there.
If you want a more robust non-blocking method for receiving serial data, see https://forum.arduino.cc/t/serial-input-basics-updated/382007