GPIO21 on Raspbery Pi is the I2S_DOUT pin, used for audio. You should pick one GPIO pin from the Pisound header of unused pins: https://blokas.io/pisound/docs/Specs/#pinout-of-pisound-header
Also, the pisound-btn assumes that the pin’s pull up resistor is configured properly already (the Pisound’s button itself is preconfigured using the device tree overlay built into the I2C EEPROM chip on Pisound), you’ll have to configure the pin you use manually: https://www.raspberrypi.org/documentation/configuration/config-txt/gpio.md, so add a line like this to the /boot/config.txt, change accordingly to your chosen pin:
gpio=27=pu # Enable pull up resistor