After upgrading your server environment to PHP 7.4, you see the following error at Help > System Health Status:

Your system's PHP version is not compatible with your current WHMCS version.

For more information on WHMCS's PHP version requirements, see System Environment Guide.

Move to a compatible PHP version for your current WHMCS release. For example, for WHMCS 7.10 you could use PHP 7.3.

For help with this process, contact your hosting provider or system administrator.