install python3 and pip3

sudo apt install python3 python3-tk python3-pip 

install fluidsynth

sudo apt install fluidsynth fluidsynth-soundfont-gm 

install pyfluidsynth with pip 3!!! NO SUDO (No upgrade...):

pip3 install pyfluidsynth 

run with python

python3 mainpiano.py 

No sound ? Test fluidsynth with a mid file

fluidsynth -a pulseaudio /usr/share/sounds/sf2/FluidR3_GM.sf2 mymusicfile.mid