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 cPanel account.
Another possible reason is a firewall on cPanel server which blocks connections of WHMCS server on port 2083 (even if WHMCS and cPanel is one server). To fix this problem whitelist IP of WHMCS on cPanel.
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.