You also need additional flags here: Difficulty with getting USB serial [USB CDC] working - #6 by maxgerhardt
But before getting USB to work you should focus on the LED blinking. If that doesn’t work it means that the controller maybe doesn’t even reach the setup() or loop() methods. Maybe stuck in clock initialization or crashing.
Can you show the code you’re using for the blink? Did you try using digitalWriteFast(PC_4, HIGH); too? Or just digtalWrite(PC4)? (Mind, one is a PinName enum, the other a integer behind a mapping)