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Two things typically cause Windows activation to fail. One is a connectivity problem, the other is a problem with...
the product key itself.
If you can't activate Windows, check the product key first. In Windows 10, right-click the Start menu and select the System command from the resulting shortcut menu. When the System screen appears, scroll to the bottom to locate the Windows Activation section.
Click Activate Windows to manually activate the operating system. This will cause Windows to open the Update and Security window, which will display the reason for the activation error. This screen also contains buttons to manually activate Windows, or to activate by phone.
If you see an error message indicating that the operating system was unable to reach Windows activation servers, and to check if you are connected to the Internet, then you probably have a connectivity issue. Try opening a Web browser and surfing the Internet. If you are unable to do so, then a connectivity problem is the reason you can't activate Windows. This problem is often related to an inability to contact a DNS server.
If on the other hand, you see an error message that simply says Windows can't activate right now, and that you should try activating again later, it's usually an indication of a problem related to the product key. The error message does not specifically point to a bad product key, but the window will contain a button for changing your product key. The very presence of this button is an indication that the product key was to blame for the problem.