Check out/Payment problem

Hello! There is a an error in your theme https://pelicanbikerentals.com/wp-admin/admin-ajax.php?action=add_bbooking_to_cart

[HTTP/2.0 400 Bad Request

User simply can not proceed with the payment if he/she is not logged in as admin. External user can not make it.

When we go further trying to understand the error, we see that there are js files inside the theme, which are responsible for sending product item to basket.
We skipped payment option at the website now because of it, but kept one page so you can test and see it. https://pelicanbikerentals.com/product/kids-bike-valencia-8-years-and-more-test/

Please help us as soon as possible.