Here is a comprehensive guide on why this happens and how to fix it. Why You See the "License Already in Use" Error
If you changed the network interface card (NIC), CPU, or hostname, the Challenge Code changes, making Tenable think it's a new device.
You uninstalled Nessus and tried to reinstall it on the same machine, but the server didn't "release" the old hardware ID.
If running Nessus in a VM, set a static MAC address. If the MAC address changes, the Challenge Code changes, and your license will break.
If you know you are going to decommission a server or move a VM, run nessuscli fix --reset first to try and release the binding.
Keep a record of which Challenge Code is paired with which Activation Code, especially if you manage multiple offline scanners.
After the license is accepted, remember that you will also need to manually download and install the latest plugin set ( all-2.0.tar.gz ) since the machine has no internet access to pull them automatically. Pro-Tips for Avoiding Future License Issues
Locate the Nessus activation code that is giving you trouble. Look for a "Reset" or "Unlink" button next to the license.
The error occurs because Tenable's license server believes the code is currently active on another machine. Common causes include:
If you followed these steps and still receive the "Already in Use" error, it usually indicates a backend sync issue at Tenable. In this case, providing your Activation Code and the new Challenge Code to a Tenable Support ticket is the fastest path to resolution.
