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