If you have a computer on which Windows 7 was pre-installed by the computer's manufacturer, that is an "OEM " system and "keyfinder" utilities will NOT show the correct Product Key for OEM versions of Windows 7. If they return anything, it will be the generic Product Key used by the computer manufacturer with Windows was installed at the factory. That key can't be used to reinstall Windows 7 or upgrade to Windows 10.
The only valid Product Key is the one on the Certificate of Authenticity (COA). You can find it in the locations given by CmdrKeene