This error can be thrown when incorrect username or password is specified in admin area for a customer in WHMCS. Username and password should be the same which customer uses to login to his account in hosting panel.

We recommend you to update builder plugin on WHMCS by documentation. It includes some features and many fixes. You can find more details on update here: https://marketplace.whmcs.com/product/1086.

This error is usually thrown when no (or incorrect) username or password is specified in admin area for a customer in WHMCS. Username and password should be the same which customer uses to login to his DirectAdmin account.

Yes, that is correct. The builder module does not have output in admin area therefore built-in WHMCS message "There is no admin output for this module" is displayed.
The module only adds button "Open Site Builder" (or "Open [your-builder-name]") in client area to each product details page of hosting type. Please log in to some WHMCS client, open details of some hosting service and you will see this button in the right side menu box.