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.
You must use the Fabrik bootstrap list template (not bluesky which is from Fabrik3.0/Joomla2.5)You can't use bluesky in J!3+/Fabrik3.1+, you must use bootstrap (or a custom template adapting bootstrap).
Fabrik is relying on the Bootstrap2 framework coming with Joomla with Protostar, so if your template's bootstrap is overriding the original one this may break Fabrik, too.My template use boostrap.
(obviously not div if filters are set to below).the only Fabrik3.1+ list templates you can use are bootstrap and div