Coupons and promotions are generally only available to users trying out a paid plan for the first time.
A "This promotion is not available for your account" error message means that you are ineligible for the promotion due to being either an active paid subscriber or having been a paid subscriber in the past.
Additionally, App Store subscribers (iOS or Android) are unable to take advantage of promotions at this time. If you are interested in transitioning from an App Store subscription to a direct subscription, you'll need to cancel and resubscribe once your current subscription term ends.