After upgrading from Joomla 3.* to Joomla 4.* you can have this error in various places:

Column 'checked_out' cannot be null

Recently during a lenghtly upgrading process of an old Joomla 3 site to Joomla 4 I have run into an unusual issue. During the process of rebuilding/rebranding the site a large number of articles where added to the original site, and obviously they are missing from the new site. I imported articles directly in the database into the #__content table like I often did in Joomla 3 - a smooth process generally. However even the database looked ok, the new articles in Joomla 4 they were not visible.