If you get this error, you have two options to proceed with selling your tickets:
You can include the memberships for free with the entrance tickets. This way, the buyer of your tickets has some added value and doesn’t have to buy a membership themselves.
You can ignore the error and proceed with selling. Memberships are valid anytime, so you can keep them if you want and use them for another event.
Going with one of these two options should make it possible for you to continue and sell the tickets!