Thanks for reaching out and welcome to the Community.
You are trying to activate the 3 months free Premium for new subscribers, correct? Please not that this offer is only available to people, who've never had a Premium account. Could you confirm that this is the case?
You should also try activating the offer from a private or incognito browser, as cached data could sometimes cause such issues and this is a simple way to avoid them. If the problem persists, it's a good idea to give it a go on a different network or with a different device altogether.
Hope this helps. Keep us posted how you get on.