This is strange. I tried your snippet with your file and it works for me:
(env) ➜ testing cat x.py
from mutagen.id3 import ID3
tags = ID3("myfile.mp3")
print(tags["TXXX:FMPS_Rating_Amarok_Score"])
(env) ➜ testing python x.py
0.78
Dict keys are case sensitive in python. In your code I can see the key you've used has a capital M in Amarok. Maybe that's the issue here