First of all, please check the issue to use the WHMCS default template. If you face the same problem, then you need to contact with WHMCS. This is because, we just provided support by customizing WHMCS default header, footer and color based on our template. We don’t touch any system files of WHMCS.
If the default template work, then make sure you don’t add any code in the wrong way by yourself in the main.js file.