If old instance exists → deregister it → reactivate new one. If old instance is dead → contact Tenable Support with proof of license ownership.
/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: nessus license already in use
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. If old instance exists → deregister it →
/opt/nessus/sbin/nessuscli fetch --deregister If successful, reactivate on the new instance: reactivate on the new instance: