Nessus licensing works on a "one code, one scanner" basis. If you are performing an offline activation, it is highly likely that you (or a colleague) attempted an first.
Once the code is reset, follow the standard offline registration steps to generate a new license file: Update License Offline (Tenable Nessus 10.11) nessus offline activation license already in use
Prevention and operational best practices Nessus licensing works on a "one code, one scanner" basis
Once the code is reset, follow these steps on your offline machine: nessus offline activation license already in use