ERROR: This product cannot be purchased.

Hello,

I have a problem with my wordpress site. I feel that it is not possible to complete purchases on products with mobile devices, and in some cases on a computer. The item is in stock and I can add it to the cart. But there will be a file at the checkout.
Error message: This product cannot be purchased. (Translated from Swedish)

I have read some other threads with similar problems, but have not been able to solve the problem with the fixes that the threads have recommended.

Has anyone had the same problem before and solved it?

The page I need help with: [log in to see the link]