Here’s a deep, technical review of the error — covering its root causes, impact, troubleshooting steps, and long-term prevention. 1. What the error actually means When Tenable’s Nessus scanner reports “license already in use” , it means the activation code (license key) you’re trying to use is already linked to an active Nessus instance registered with Tenable’s licensing servers.

Would you like a template for contacting Tenable Support to expedite the license release?

If old instance exists → deregister it → reactivate new one. If old instance is dead → contact Tenable Support with proof of license ownership.

Keep reinstalling Nessus hoping it works – each new install generates a new instance ID and will keep hitting the same error until the original registration is cleared.

/opt/nessus/sbin/nessuscli fetch --challenge If this returns nothing or an error, the instance is not activated. Try deactivating the current (old) instance if it still exists:

/opt/nessus/sbin/nessuscli fetch --deregister If successful, reactivate on the new instance: