PC14 and PC15 on STM32F401 not working as inputs
One of the issue I find is the jumper settings  

By default, both jumpers SB48 and SB49 are open. 



Reference: Schematics of Nucleo F401RE

Second way is to later measure the resistance from the resistors highlighted and
connector CN7 pin numbers 25 and 27.

If the hardware settings is fine, you should read zero ohms approximately.