You could also try manually reactivating by going to Settings > Update & Security > Activation and clicking "Troubleshoot" again, but choose the option that says "I changed hardware on this device recently."
This happened to me after an upgrade. Running the activation troubleshooter and selecting "I changed hardware on this device recently" finally fixed it for me.
You could also try manually entering your Windows 10 product key in the activation settings. Another option is to contact Microsoft support directly, as they can sometimes push the activation through on their end.