Hi Achintha,
Thank you for posting, I am here to help. Activation is a technical process that pairs the product key or digital entitlement with the hardware configuration of the device.
A Windows license is associated with your device hardware. Significant hardware changes, like replacing the motherboard, can prevent Windows from locating a matching license when the device restarts. If Windows came preinstalled on your device and you replaced the motherboard, you may need to purchase a new license.
May we know the exact error message you are getting when you enter the product key.
Please post it here. This will help us further check the problem.
looking forward to helping you out. If you need more help please let me know. If the answer is helpful, please click "Accept Answer".
Best regards,
Jansher