We Looked Everywhere For This Page! (Error 400)

i built a wix site for a customer and when a customer completes buying a product it brings them to a sign-up / sign in page. To the top right corner there is a X (close page button) and when you click on this it brings the customer to a 400 error page. This is very confusing for the customer as they are not sure if the transaction went through and i cannot seem to find where in my dashboard to be able to change/edit

this or direct them to another page. Please help!!!

This is a forum for Wix Velo . Please direct your query to Wix Support as we are not equipped to help you with such queries on this forum.