MarketPress default "Buy now" button not working with Twenty Thirteen Child Theme

The “Buy now” button of my product pages is not adding any item to the cart after clicking it. It reloads the page and returns the message:

“There are no items in your cart - go add some !”

During the chat we ran a plugin/theme conflict test and found that the issue was not there if using a default theme like Twenty Seventeen, but it is if using the current theme, which is a child theme of Twenty Thirteen theme. I wonder what happened with the theme.. it has worked for years :slight_frown: