On newer Windows versions, the option to "enter serial number" may be unavailable due to permission restrictions.
The registration tool may not be included in some downloads or might be flagged and deleted by antivirus software like Malwarebytes .
In some cases, the registration fails because the app cannot write to the HKEY_LOCAL_MACHINE registry hive. Use the Windows Registry Editor with caution to verify permissions for the Syntrillium keys.