I encountered the same issue upgrading from 10.15.2 to 12.5.
"diskutil list" showed my volumes were already APFS.
Solution for me was to do:
diskutil apfs updatePreboot /Volumes/Macintosh\ HD/
and then reboot (not sure if that was necessary) and then Preferences -> Software Update -> install Monterey.
Thanks to the contributors to this thread which helped point me in the right direction!
benissimo
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