Nessus phones home to Tenable’s license servers. If the key is invalid or revoked, the scanner will revert to Essentials mode or stop working entirely.
You can get a legitimate activation code by registering on the Tenable website Nessus Trial: nessus license key github
Searching for a is a common path for those trying to bypass the high costs of professional vulnerability scanners. However, this search often leads to significant legal, security, and functional risks. The Reality of "Nessus License Key" Repositories Nessus phones home to Tenable’s license servers