Skinnymoonhunter this is a known issue that will be fixed in the next beta https://github.com/flarum/core/pull/1580
However unlike the name suggests the uninstall button only rolls back database migration and doesn't actually uninstall anything. This extension doesn't have any migration, so it would have no effect anyway.
To remove the extension package, use Composer.
composer remove s9e/flarum-ext-mediaembed