Nessus Offline Activation License Already In Use -

The most reliable solution is to manually reset the license association on Tenable’s activation portal.

If the license reset on the portal does not work, Tenable may have flagged your activation code due to repeated failed attempts. In this case: nessus offline activation license already in use

This is less technical but happens frequently with the "Nessus Essentials" (free) vs. "Nessus Professional" (paid) versions. The most reliable solution is to manually reset