For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Did either of you upgrade from 1.5 or was this a clean install?
If you did upgrade did you use jupgrade?
Can you try:
"Refresh Cache" in ExtensionManager/Manager (on all fabrik plugins)
then "Purge Cache" in ExtensionManager/Update
(and "Purge Cache" in ExtensionManager/Discover but this shouldn't affect the displayed updates)
Did you do a GitHub update and install additional plugins?
Those can't be updated via Joomla Updater, you'll have to update via GitHub again (and then refresh/purge cache in all Extension Manager tabs).
The main thing is to "Refresh Cache" in ExtensionManager/Manage on all (noncore) fabrik plugins.
Because a Github update is just copying files (including xml with new version numbers)