Win 10 is digitally activated so it may have problems connecting to MS servers.
However, in Win 7 when the not genuine error popped up, it was found that resetting the licence store would resolve, otherwise you may need to contact MS for them to activate it for you.
To reset the Licence Store, open a Command Prompy (Admin) and enter these cmds -
net stop sppsvc
(wait until the service has stopped before entering the following lines)
CD %windir%\ServiceProfiles\NetworkService\AppData\Roaming\Microsoft\SoftwareProtectionPlatform
(you can copy & paste this one to avoid typos)
REN tokens.dat tokens.bar
net start sppsvc
slui.exe
After a couple of seconds the Windows Activation dialog will appear.
You may be asked to re-activate and/or re-enter your product key, or Activation may occur automatically.
If you are asked for your Key, use the one on the COA sticker on the machine's case.
Reboot.