If your old server is dead, decommissioned, or lost, you must release the license lock on Tenable’s side.
: When you activate Nessus on a system, that system becomes the "active" instance. Subsequent attempts to use that code on another machine will trigger the "already in use" message. How to Resolve the Error
The "already in use" error means Tenable’s servers believe your specific activation code has already generated a license file for a different Challenge Code. You are trying to generate a second license file for a second machine using the same activation code.
This is less technical but happens frequently with the "Nessus Essentials" (free) vs. "Nessus Professional" (paid) versions.