quick workaround
create also Soundtracks.sh in same folder of Soundtracks.py with this line
kodi-send --action='RunScript("path/to/your/soundtracks.py")'
then add favorite manually like this
<favourite name="soundtracks">System.Exec(/path/to/your/Soundtracks.sh)</favourite>
tested - working
you can also try this might also work - not tested
Edit: you can probably also start your playlist from the soundtracks.sh by changing line to
kodi-send --action="PlayMedia(/some/path/to/a/file/Soundtracks.xsp)"
I tried executing it from ssh shell - this fails with the error above.
You can't do that it won't work as you need to run it internally so for testing in ssh you run
kodi-send --action='RunScript("path/to/your/soundtracks.py")'