I cannot install new updates on this staging version and WPML settings page gives me a fatal error.
I also try to find a place to install the product key for this testing version but was unable to find a place to insert that
There's a big chance that what you are seeing is a false-positive that remained in the server caching and the notification will be removed automatically when the cache will automatically clear on its own.
To check if that's a false positive, go to Plugins > Add new > Commercial > click the "check for updates" button and if the page refreshes and WPML is still showing as registered, then yes, that is just a false positive.
