It's a bit of a guess because I don't know the youtuber plugin well, but it does not uninstall correctly. I remember a similar problem we had with the Breadcrumb NavXT Uninstall routine and a similar error (the function-name was naturally different). I could handle that for MU setups that time by providing a uninstall.php with the plugin. It must be still in.
Please check that you have properly deactivated the youtuber plugin and the uninstall of it at least one-time was requested. After that, check if the plugin is still in the plugin directory. If so, rename the folder of the youtuber plugin. This should solve your issue.
You can delete the directory as well if you don't need it any longer, renaming often is faster and easier for testing. As always: Backup first.