The US sales tax calculation in X-Cart 5 can be enabled using the Sales Tax add-on. Typically you will find this add-on already installed and enabled in your store. (If this is not the case, you can install it any time from X-Cart App Store following the add-on installation procedure.)
If the Sales Tax add-on is installed but not enabled, you will need to activate it.
Please find below some guidance on how to set up and enable tax calculation using the Sales Tax add-on.