For systems without direct internet access (air-gapped networks), Nessus cannot activate automatically. You must use the offline activation method, which generates a challenge code on the scanner and then obtains a license file from Tenable’s portal using an internet-connected machine.
Here’s a draft text explaining how to perform an offline activation for Nessus. nessus offline activation
You can use this for internal documentation, a knowledge base article, or an email to a client/team. How to Activate Nessus Offline (Air-Gapped / No Internet) a knowledge base article