Products can not be added to the cart

Products can not be added to the cart

Hi there,

I am configuring a new commerce site, but I am not able to add any product to the cart. When I click the "Add to Cart" button, the loading indicator appears but nothing else happens.
I inspected the network activity using the Developer Tools on Chrome and I realized that every time I tried to add a product to the cart, I am getting a 400 error. Here, it is a screenshot.

 
In addition, when I click on the Cart Button on the Navigation Bar, I am getting a blank page.

The commerce is on preview mode and has the offline payment setup.

I appreciate it if you can give any guidelines on this.
Thanks.