serial number not valid why?

Make sure it's entered correctly. From "About This Mac" dbl-click on the Serial Number, press "Command + C" then paste it into the form. Sometimes it's hard to tell the difference between a 5 and an S, a O from a 0, etc.

If that doesn't work, you need to contact Apple Support.

Did you perhaps have the motherboard replaced at anytime? That often changes the serial no. and unless is reset properly will cause issues like this.

If that's the case you'll need to have Apple reset the S/N correctly.