Installing j2commerce on a joomla4 website generates a fatal error 'class F0FDatabaseDriverJoomla extends F0FDatabase implements F0FDatabaseInterface' etc. see attached screenshot.

OK I've bitten the bullet and am restoring a backup of the last time the site was J3 (having taken detailed notes of the changes I had made

OK; I restored Joomla 3, followewd the rules in your migartion instructions, including using the Magic Eraser to remove the duplicate fof folders and disabled all the plugins and successfully upgraded to 4.4.14. I now get the message:

0 Class "F0FModel" not found

with no other information; the only fof file that exists is fof30, mostly dated Sep 2018 with a few 2021;  is there a fix for this or does it mean I have to re-run the whole backup restoration process?

I note that the error log shows the following the J4 upgrade: 

"[01-Oct-2025 16:33:16 UTC] PHP Warning: include_once(..../libraries/f0f/include.php): failed to open stream: No such file or directory in .../plugins/system/j2canonical/j2canonical.php on line 13"

I have a back up taken shortly before the 'Magic eraser' clean-up. Presumably restoring that would fix this?

I disabled the two items that the joomla update said were a problem and then ran the update it looked OK initially but on attempting to enable the Community builder as suggested on the Dashboard, I get the error 

Attempted to load class "J2Store" from the global namespace.
Did you forget a "use" statement?

One of the tracks is in the attachment; (sorry; the file got rejected due to including spaces in the name); here's the first lines:

".../plugins/system/j2canonical/j2canonical.php (line 40)

1.     }
3.     function onBeforeCompileHead () {
4.         $app = JFactory::getApplication();
6.         if (J2Store::platform()->isClient('administrator')) {
7.             return;
8.         }

Brilliant; many thanks for this; I've done what you suggest, it worked and I will now update the J2Commerce as you suggest.

OK I updated the installation, enabled the plug-ins and now get an error ' Class "JRequest" not found ' which has broken the site again

the front end now shows: (php 8.2)

Deprecated: Using ${var} in strings is deprecated, use {$var} instead in /.../libraries/f0f/less/less.php on line 2428

Deprecated: Using ${var} in strings is deprecated, use {$var} instead in /...libraries/f0f/less/less.php on line 2428

Deprecated: Creation of dynamic property plgSystemJ2pagecache::$_cache is deprecated in.../plugins/system/j2pagecache/j2pagecache.php on line 46

logging in to the front end shows a long list of 'deprecated' errors all involving; J2StoreTableProduct:: eg:

Deprecated: Creation of dynamic property J2StoreTableProduct::$j2store_product_id is deprecated in .../libraries/f0f/table/table.php on line 645

For the time-being, I've set the error reporting to 'none'

I resolvedthe Request error by editing the code at j2xml.php line 60 to accord with the php 8.2 requirements and the site now seems to be working front and back

Many thanks for your help with this and the prompt resonses; these issues have been resolved and I can move on to the next ones...

And thanks to you all for taking over J2Store, which was perfect for my requirements, especially the Royal Mail plug-in