Trouble deactivating Trados 2017 before activting apgrade to 2021

I have purchased an upgrade from studio 2017 to studio 2021 and want to activate the new version. Apparently I have to deactivate 2017 first, but when I try to do this I get the following error message: Login for this product key has been restricted. The 2017 version is no longer visible in my SDL account, so I can't request an offline deactivation. Can you please advise what I must do next? Many thanks, Alice