This is a barely educated guess.
In Picard/Options/General there is “Musicbrainz Account” - have you entered your Musicbrainz account* details there and then collected the authorisation code from the new tab that opens in your browser and then cut and pasted that code back into Picard?
When I do that I end up with;
Logged in as mmirG
Please report back if this works.
I searched in the documentation for “Please select a valid fpcalc executable” and if it is a “simple fix” then it would be helpful for newer users to have the phrase indexed in the documentation.
*(I’m 99% confident that it is the same account as you are using on these forums.)