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It sounds like you tried to restore a Joomla 5 site on top of a Joomla 6 site. This will not work. You have to delete the files of the Joomla 6 site (NOT just the administrator directory!) and restore over what is essentially a blank site. The only directories you can reasonably keep are images and files (the latter only if it exists), as well as any other top-level directories you have created yourself. All other files and directories must go.

Do note that this advice appears twice in the documentation, first here as an aside towards the end of this paragraph:

Restoring a backup does NOT delete files on your site which don't exist in your backup. If a file exists on the site you are restoring to but not inside the backup archive itself it will not be overwritten. This is important in two cases. First, when you are restoring a backup after your site is hacked (unhacking a site). The hacker may have left behind files which can be used to re-hack your site. These files will not be deleted automatically. Use a component, such as Admin Tools Professionals with its PHP File Change Scanner, or a third party service such as Sucuri to detect and remove such files. Furthermore, if you are trying to replace a site with a new one. If the old site is based on an older version of Joomla!, a different CMS (e.g. WordPress), is a static site or had different extensions / templates installed these files will be left behind. In both of these uses cases you should take a copy of your site's files, then delete all files and folders, create a new database and finally restore the backup.

and explicitly stated in this paragraph:

Do not try to overwrite one Joomla! version family with a different one. Overwriting a major version with another (e.g. restoring a backup taken on Joomla! 3.7 on top of a site running Joomla! 2.5 or vice versa) or between different minor versions (e.g. restoring a backup taken on Joomla! 3.7 on top of a site running Joomla! 3.6 or vice versa) will NOT work. Joomla! moves files around between minor and major versions. Since the backup does not delete files not present in the backup archive this will end up with Joomla! being "confused" and malfunctioning. In these cases you should delete the existing files and folders (except, perhaps, user generated content) before restoring the backup. You can safely restore a sub-minor (path-level) version on top of another. For example, you can safely restore a Joomla! 3.7.5 site on top of a Joomla! 3.7.3 site or vice versa.

This advice has been in the documentation since 2011 because it's a recurring question.

No worries! Thank you for being understanding :)

You're welcome! I am happy I could help :)