I cannot add the preorder product to cart

If you are seeing an error when trying to add a preorder product to cart, such as "variant can only be purchased with a selling plan" or "selling plan does not exist", or something similar, this is typically an easy fix.

Go to the product page in your shopify admin and go down to Purchase Options. Uncheck the box that says "only sell this product with these purchase options". That should resolve the issue. If this doesn't work, contact support and we'll fix the issue.