I have Use AcoustID selected and fpcalc installed with an API Key.
An AcoustID tag is generated when I Scan a track that doesn't already contain one. Using either Lookup or (more often than not) Lookup In Browser to find the matching release, and with the track moved to the album in the right-hand pane, I Save it and confirm the metadata is updated and the file renamed and moved from my untagged folder to my tagged folder.
I then select the release in the right-hand pane and click Submit AcoustIDs, then check the Fingerprint tab for the track under the correct release in the MusicBrainz database and see nothing. I check back several times (over an hour later at this point) but the fingerprint for that track is still empty.
The Submit AcoustIDs button appears to work (i.e. it displays as being clicked and is not ghosted) and there is no error message, so I don't know if the problem is with Picard, fpcalc, or the MusicBrainz database website.