Hi AyKayo,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
May I know where did you get the Product key? Was the Windows activated once you bought it?
Windows Activation depends on what type of license do you have. If your computer came preinstalled with an OEM license, the license was tied with your old motherboard and it would not be transferred. If it was a retail key try to reactivate it:
Settings > Update & Security > Activation > Change Product Key and enter the key that you have.
If it is a digital license linked to your Microsoft account, it should activate the Windows automatically after signing in with your Microsoft account.
If did not work, try to troubleshoot it by going to Start > Updated & Security > Activation > Troubleshoot.
Get help with Windows activation errors:
https://support.microsoft.com/en-us/windows/get...
Please let me know if you need further assistance.
Stay safe