On the Subscriptions screen, a subscription shown as Unassigned isn’t currently applied to any group.
Tap Assign to apply that subscription to a group. Once assigned, that group will be covered by the subscription (and the group should stop showing paywalls like “Group needs a plan,” if that was the issue).
If you don’t see the group you want to assign to, make sure you’re a member of that group on the same account, then try again.