To accommodate old license holders, Nitro transitioned many version 11 licenses to work with Nitro Pro 12 (specifically build 12.16.9.574) How to Activate a Legacy Serial Number
Nitro recommends upgrading to the latest build of Nitro Pro 12 (specifically version 12.16.9.574 ). Existing Nitro Pro 11 serial numbers are often compatible with this specific version 12 build.