How to add an "add to cart" button

hi there,

on my website, I have a product page, for physical and digital products.
Besides that, I also have a page for our courses. This page has 2 products, and I want to add an “add to cart” button to each of these two courses.

However, I get the message:
Schermafbeelding 2022-08-03 om 09.40.23

How can I fix this?

Here is my site Read-Only: LINK
([how to share your site Read-Only link][2])