Since v1.2 BlackPill F401CC/F401CE boards have an user button (named "KEY") connected to PA0 added to the board. You can find more details about this boardhere

Please add the USER_BTN define mapping to PA0 for BlackPill F401CC and theproposed F401CE boards. That would be consistent with the already existingBlackPill F411CE USER_BTN mapping

I believe the following change in theGeneric_F401Cx/variant.h file should do the job:

// On-board user button
#if defined(ARDUINO_BLACKPILL_F401CC) || defined(ARDUINO_BLACKPILL_F401CE)
#define USER_BTN PA0
#elif !defined(USER_BTN) && !defined(ARDUINO_GENERIC_F401CC)
#define USER_BTN PC13
#endif