Hello,
Ok, so we have further checked this concern on your Site and deactivated the remaining third party Plugins which were active on your Site and then checked back the Add to Cart functionality for the Products along with further going to the Cart and Checkout page. The functionality was then working fine after we deactivated the remaining third party Plugins.
So, it is a third party Plugin conflict on your Site not an issue at the Theme end. You need to reactivate the remaining ones one by one to see which Plugin was causing the conflict in this regard.
The following Image shows those third party plugins on your Site which were active before deactivating: https://snipboard.io/wImYxM.jpg
So that it would become easier for you to find the one causing the issue.
Best Regards.