Nitro no longer supports version 11 or older. Users attempting to activate this version typically encounter error messages because the software can no longer verify the key against the deactivated servers.

Searching for "high quality" activation keys or "keygens" on third-party sites carries severe security and legal risks: Activation - Nitro Pro

Nitro Pro 11 licenses not upgraded as promised - Activation/Installation

Run the installer as administrator. Accept the license agreement and choose "Complete Installation."