I just got a gift card and typed the code in the link but it didn't get me premium. How do I use it?
Did you type it in at https://www.spotify.com/uk/redeem/ ?
If so, did it accept it and did you check that you'd typed it in right (eg. you may have read capital i as lowercase L?) and that you'd typed your account details in correctly?
If you're sure you did all this right, I'd contact Support.