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
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We have resolved the issue with the J! updater and this will be fixed in the next release.
But anyway...Front end testing templates were Cassiopeia and to a lesser extent Helix Ultimate. Admin testing template on Atum. At this time we do not want to get into any issues with other templates.
As I assumed: Obviously you have a Fabrik3 override in your Gantry template.1 () JROOT/templates/g5_hydrogen/html/layouts/com_fabrik/element/fabrik-element-label.php:7
If it's this error it's an override.1 () JROOT/templates/g5_hydrogen/html/layouts/com_fabrik/element/fabrik-element-label.php:7