If a customer isn't prompted to complete a required Purchase Agreement when purchasing a Membership or Credit Package, the Purchase Agreement might not be active at the locations where the product is being sold.
While the Membership or Credit Package is linked to the Purchase Agreement, the actual Purchase Agreement might not be configured to be used at a certain location.
Go to Configuration > Agreements and find the Purchase Agreement. Under Locations, select all the Locations where the product is being sold.
If the Location is not added here, any Memberships or Credit Packages sold at those Locations will not be prompted to sign the Purchase Agreement.