DFPlayer library using RT TX pins
On a classic Arduino pins 0 and 1 are used for serial communication with the host PC, so you cannot use them to talk to a peripheral without conflicting with that.  The library may well be coded to trap and reject such an attempt.

On various alternate (ARM, Atmega32u4, etc) boards these pins may be usable, though the library may not be aware of that.  Also on such boards you would generally want to use an available hardware UART rather than a software simulation of one, which may be fairly ATmega-specific anyway - the end result of which is that library code may well need modification to work on a non-traditional device.