Make sure your OSX S/N is valid but not registered ...
I recommend that you check any existing or newly generated OSX S/N using Apple's Self Solve website which can be accessed at the following URL:-
https://selfsolve.apple.com
Simply copy and paste your OSX S/N into the form and click on continue, if your S/N passes the validation checks and is not registered then site will return with the following message:-
We're sorry, the number you have provided cannot be found in our records. Please verify the number and try again
This is good and is the only message you really want to see ..... it means your OSX S/N does not match an existing Mac's S/N but it does pass all of Apples S/N validation checks, this is also useful when calling Apple support as the S/N will appear valid but will not show up on their systems .. in-most cases making them think the issue is with their registration database and not your OSX S/N.
Finally, if you get the message :-
We're sorry, but this serial number is not valid. Please check your information and try again
Then the S/N is either black-listed or failing the OSX S/N validation checks, do not use it - generate new S/N and try again.