The "Nessus offline activation license already in use" error occurs when the activation code you are attempting to register is already linked to another Nessus scanner instance . Tenable licenses are generally restricted to one active scanner at a time; if you attempt to use the same code for a new offline activation (e.g., after a hardware failure or server migration) without first unlinking the previous one, the system will reject the request. Core Reason for the Error
The system will output a unique, one-time challenge code. nessus offline activation license already in use
Restart the Nessus service afterwards to ensure the license registers seamlessly: The "Nessus offline activation license already in use"
Complete the installation by updating the plugins: nessus offline activation license already in use