I assume you're using the same account you purchased Silent Hunter 5. If not, this is the correct error page if you are trying to register the game under a different account.
I had the same issue, where I was using a newer Uplay account to install the game. This will not work. You have to use the original email account associated with the game, regardless if you have the CD Key.
Once you contact them, you can transfer the registration from the old account to your new account and it takes about 24 to 72 hours until that is complete.