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This issue is not related to our software. The referenced table is part of Joomla! itself. Moreover, creating menu items is handled thoroughly by Joomla!. Our software is not involved in this process. I'm also quite sure that the same issue will occur when you try to create a different menu item, create or save an article etc. Basically, you've hit a problem with Joomla! itself.

While I cannot provide support for Joomla! itself, I can point you to the right direction. UCM is part of Joomla! itself and stands for Unified Content Management. This is used in Joomla! (NOT in our software!) to link together different items and provide history, tags, custom fields etc. The UCM system has its own database tables. One of these tables (#__ucm_content, where #__ is the database table name prefix you are using on your site) is either broken in the database or missing.

If you have a recent backup of your site you can restore it on another server (e.g. a local XAMPP installation) and then use phpMyAdmin to copy the restored #__ucm_content table to your site.

If you don't have a recent backup, you will have to seek support for Joomla! itself in the official Joomla! forum (joomla.org), the official Joomla! Facebook page or a Joomla! forum in your language. Someone there should be able to tell you how to reconstruct the #__ucm_content table. It typically involves pulling out commands from the joomla.sql file shipped with the Joomla! installation package, modifying them a bit and running them on your site with phpMyAdmin.

I hope this information helps!