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
Please see our announcement here.
Do you really have the Fabrik content plugin enabled?
BTW: as a Standard subscriber you should post in Standard forum (community forum is last in the queue for support)
Next time you should better mention such setups like "using Fabrik content not in an article but in a custom HTML module"Solved, I forgot to set "prepare content" in options html module