I have encountered this problem - Info: To add this product into your cart you have to select a product variant on the details page. I found the problem. On my website we add personalizations which is a custom attribute. In the custom attribute if this left empty it won't be added to the cart. Howerver, if you type in any character it allows you to add to the cart. So I want to have text already in the customer attribute box so there is no problems.