The reason why you are getting Windows Activation issue is due to the changes on your Hardware which is the Motherboard.
Windows 10 now have Digital Entitlement wherein the product key will be encrypted on the Motherboard, this can be obtain through the free upgrade.
And since the Motherboard was replaced, the server still recognize that the product key is still encrypted on the old Motherboard.
Your OS is genuine. It still shows that Windows is installed on older Motherboard and in use, hence it did not accept the product key.
You can try online and manual activation and see if that works.
Online Activation - go to Command Prompt Admin (right click on windows icon and click command prompt admin ) type slui 3 and press enter and put in your Product key.
Manual Activation- go to Command Prompt Admin (right click on windows icon and click command prompt admin ) type slui 4 and press enter select your location and contact the number provided.
If all else fails, option you can consider is to install previous version and upgrade to Windows 10 again to obtain another digital entitlement.