The activation code must be on a sheet of paper with a red blob in the right corner that say’s “STOP Don’t throw away!” (in fact - you should have several sheets that contain demo versions of other programs like Halion Sonic as well). If Cubase is already activated you should see a license that has a unlimited time frame (sometimes you see the demo license as well).
What does the eLicenser Control Center say is on your USB dongle? To view - Open up your control center and look what license is on the USB-eLicenser (click on the picture of the dongle and read what is said on the right side of the window).
#Cubase license code paper in box full version
Keep in mind the full version of Cubase cost a lot of money (€ 559 for the Pro version), so if you (or the one that bought it) payed a lot less, it is probably suspicious. If this is the case I am afraid Steinberg can do nothing for you (Cubase cannot be activated without an valid license number), and you have to get that valid license number from the seller him/her self. If that box was unsealed and it has no license paper inside, I am afraid that license is probably been stolen, or the seller has sold only a box with disks and USB key without license, a thus has cheated the buyer. In this case I hope your package was still sealed when you got it, because there sure has to be a sheet of paper inside the box with a license number on it.
Did you registered the key with your Steinberg account? In that case it is possible you run in demo mode, and it will switch to full mode after the demo is expired. The Cubase 9.5 you got is second handed, and the license is already on the USB key. You do not say witch version (Elements, Artist or Pro) you got, so I assume it’s about Cubase 9.5 Pro.