When I try to add the membership to my cart, I am getting the error "Variant can only be purchased with a selling plan". What do I do?

If you're seeing the error "Variant can only be purchased with a selling plan" on your membership product page when you try to purchase a membership, it usually means that the selling plans haven't been added properly to your theme, so Shopify isn't able to find the subscription payment option associated with the membership tier you're trying to purchase.