This information is provided on a sticker on the bottom of your NUC (as well as on a sticker on the SODIMM bracket inside your NUC). It should be specified exactly as it is provided on the sticker. For my BN NUC, the sticker says my serial number is G6BNxxxxxxxx and the S/A number is J31153-302. I entered this information exactly as it appears on the sticker. Note that, in the portion of the Serial Number that I have x'ed out, you will see only zeros (there are no letter o's in these strings).
Hope this helps,