If you can't purchase Premium, it's likely that your album is already subscribed to our Premium service. Only one Album Admin (parent member) of an album needs to purchase a subscription for all members of the album to get access to Premium features, which means that your album can only have one active subscription at a time.
If you've recently cancelled your subscription and can't purchase Premium, this is likely because your Premium subscription hasn't ended yet. Canceling your subscription stops your Premium subscription from automatically renewing at the end of the billing cycle. This means that even if you've canceled your subscription, you won't be able to purchase a new subscription until the current subscription ends.