If the alert reminding that a course, service or multipack requires a valid membership fee pops up and the customer already paid the membership fee, there is no bug nor error: the system requires a valid membership fee at the time of sale and most probably the fee the customer paid for has a validity that starts in the future. 

Example: the membership fee the customer already paid for is valid from September. If the customer wishes to purchase a new mutlipack or enrol onto a course in August, the system will suggest selling a membership fee since the customer doesn't have a valid membership fee for the month of August. 

To avoid this situation, it is possible to create a season for the validity of the membership fee that starts from August even if the official season for courses and subscriptions starts in September.