In this issue, you need to contact with the WHMCS team to get support. Because that functionality built by them and we only customized their SIX template’s header, footer, and color to match our template, nothing else.
I wrote Whcms and they checked and that’s what they wrote:
To test this issue further, I added ?systpl=six to the end of the URL. This will force WHMCS to use the stock Six template we ship with a fresh installation. When WHMCS loads this template, everything works correctly which indicates your issue is a result of a custom or out of date template.
To resolve this issue, you will need to contact your template developer and ask them to ensure WHMCS is updated for the version of WHMCS you are running. Each time we release a new version of WHMCS, we make a note in our Release Notes of any changes to our stock templates.