Internal Error: We do not know what version you want to upgrade to, and there are no stable versions in the system (which we use as the default). Please generate an upgrade file from https://composr.app, in the news article for the newest release. You can then use this in the upgrader when transferring across new / updated files. You may have to re-load the upgrader manually by going to baseurl/upgrader.php so you do not get this error again.