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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My basis joomla 2.5.14 (last update) , kunena 2.0.4 , fabrik 3.0.8
I have used exactly the tuto
1) Upload latest Fabrik version (to be downloaed from Git Hub - https://github.com/Fabrik/fabrik/zipball/master)
I have extracted all files and uploaded by ftp (filezilla with files replacement) relevant joomla folders : administrator, components, libraries, media, modules, plugins, on the root of my web site
Not done on first tests
2) Set "Fabrik - System" plugin in the #_extensions table, and set 'ordering' to -1.
to be sure to have no pb with other system plugins, on joomla administration plugin, i have put the kunena system plugin to the latest place
3) Empty Cache (site and administrator by FTP - I had only index.html file on each cache folder )
4) Install Kunena 3.0 package (directly from joomla 2.5 extension administrator update)