Changes to the network adapter, MAC address, or hostname can make Nessus see the same machine as a “new” one.
: Select your product type (e.g., Nessus Professional) and click Manage Product next to the specific license. Perform Reset : Click the Reset Product Activation link.
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.