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.
If you have a non-bootstrap Joomla template you may try an extentions loading the bootstrap, e.g. http://digitaldisseny.com/en/joomla-extensions/twitter-bootstrap-plugin-joomla (not tested)