We’re sorry, but this serial number isn’t valid. Please check your information and try again.

I assume you are using the serial number you find when you click on the About this Mac under the Apple menu? If so, call AppleCare in your region. I had this happen with a new iPad and the AppleCare rep took care of it right away.

If you did not purchase the laptop directly from Apple or an authorized partner, then the laptop's purchase won't be registered with Apple. Even if purchased directly, sometimes the information won't be on file either.

You could take your laptop to the Apple Store or an Apple Authorized Service Provider. In some regions Apple techs will use the date of manufacture to determine coverage. It doesn't hurt to see what they say in person. You could also try asking the place you purchased the computer for a copy of the receipt.

Copy and paste the serial number from the "About this Mac" screen accessed by clicking on the Apple menu.

The system serial number is also located on the bottom of the laptop, but it is very easy to confuse numbers and letters. Apple only uses zeroes in their serial numbers and never uses "ohs".

MBPro 2016+ Keyboard Repair Program:

https://support.apple.com/en-us/keyboard-service-program-for-mac-notebooks

List of current publicly acknowledged free repair programs for various Apple products (some have multiple programs available):

https://support.apple.com/exchange_repair