You have obviously purchased the upgrade from some earlier version of Lightroom, and a serial number from the earlier version is required where it is asking. Do you have the serial number from an earlier version of Lightroom?
Thanks for your reply!
No... this was my first purchace on adobe, and it stills the first registration on my plans and products list.
I now own three diferents CC acounts, but I need to use this first purchase to teach my youngest.
Is there another way to activate this serial number? Even with an older version of lightroom?
If you bought the upgrade by accident, when you should have bought the full version of Lightroom, I don't know that there's anything you can do at this point; Adobe does not sell Lightroom 5 any more so you can't buy the full version.
I don't know if there's anything you can do at this point, you'd probably have to discuss this with Adobe Live Chat. Contact Customer Care