Add The Wire As Your Trusted Source 
On the offline Nessus server, open your terminal (command prompt) and run:
Generate a unique, one-time challenge code for offline registration of a Nessus scanner. This challenge must be submitted to Tenable's support portal or offline registration system to receive a corresponding license or activation code. nessuscli fetch --challenge
The nessuscli fetch --challenge command is a specific feature of Tenable's Nessus vulnerability scanner. It's used to generate a required when registering a Nessus scanner in an offline (air-gapped) or proxy-restricted environment. On the offline Nessus server, open your terminal
: On a computer with internet access, navigate to the Nessus Offline Registration Page . On the offline Nessus server
The basic syntax for using nessuscli fetch with --challenge is as follows: