Same error here: I can run my brew installed python3 but the version shipped with Zerynth is broken on Big Sur.
I fixed it by making a symbolic link of the python folder inside
/Applications/Zerynth Studio.app/Contents/Resources/app.nw to my local python folder:
mv /Applications/Zerynth Studio.app/Contents/Resources/app.nw/python Applications/Zerynth Studio.app/Contents/Resources/app.nw/python_old
ln -s /Users/xxx/.pyenv/versions/3.7.3 /Applications/Zerynth Studio.app/Contents/Resources/app.nw/python
The installation completed but now I am having some weird behaviour in the Zerynth Studio and does nto seem to work properly.