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
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
what do u mean by mask and how can it help testing the phone format ?There's also the possibility to define a "mask" in the field element "Formatting" settings.
how can i use the regex validation ? is it available in fabrik's elements ?You can use regex validation, but if you want to allow formats from all countries there are endless permutations to consider. Google: regex telephone number validation
do i need to activate the ajax validation ? also in the condition it asks for php code, but the phone testing isn't it done by Javascript ?Element -> Validations -> Action -> regex