This error occurs due to the fact that your account has an active subscription, wait for it to end, and try to use it again.
If you don't have an active subscription, maybe the charge is active, it's trying to renew and it's being declined by your card. You must cancel this charge to activate the code later.

You will receive the code shortly after the payment is approved via email and it will also be saved here on the website within your account area.