client got error after paying on WHMCS

My client ordered a service, went through the process of paying successfully and got this error after paying:

Sorry this doesn't look like a WHMCS site at [https://www.nocohosting.com/whmcs/] Make sure your WHMCS Integration settings are pointing to the correct URL and that the WHMCS site is set for the Portal template in Setup | General.

No settings have changed, Portal is being used in WHMCS.