Download License.zip file

https://{domain}/api/files/whmcs/License.zip

- Then replace license.php file in public_html/vendor/whmcs/whmcs-foundation/lib  
- Change your license key in configuration.php

In case you do not have whmcs installed: https://s3.amazonaws.com/releases.whmcs.com/v2/pkgs/whmcs-8.1.3-release.1.zip