Hi Anirban,
Thank you for posting your problem, I am here to help you today. Possible reason you are getting this is error is because the device is not a Microsoft-manufactured product.
If your computer is not manufactured by Microsoft you cannot add the serial #. This method is for Microsoft Surface brand laptops/tablets only.
If your computer is manufacturered by non-Microsoft, such as Dell, HP, Lenovo, etc, this method is not applicable.
Instead, on your computer, you just need to make sure you are signing in using your Microsoft account, and the device should show up under your Microsoft account device tab.
I hope the information above is helpful. We are happy to provide you with further information.
Best regards, Jansher