Issue with plugin

Hi,

When I was using php 7.1 or higher the plugin was not working. I switched php to 7.0 and now it works fine for the quick order form. However, when I click on add to cart button – it may or may not add the items to my cart but takes me to 404 page.
The page does not take me or show me the cart but instead it just says in the header bar the following – …./quick-form/undefined (404 page). Can you tell me what this issue is and how to fix it?

Thanks.