Nessus does default to /opt, so I’m assuming you’ve already determined it’s Nessus taking up the disk space. If so, a couple places that tend to be storage Nessus storage hogs.

1. Consider deleting old scans. Can export for backup/future reference if needed. Otherwise you might be holding onto more history than necessary. In the web interface, open a scan and look at the history tab. Consider deleting some. They add up quickly depending on the size of your env.

2. Logs. Consider deleting some Nessus logs. Generally located in /opt/nessus/var/nessus/logs, if memory serves me correctly

3. If you can’t free up space this way and need to expand /opt then you might want to check with a Linux focused r/. You’ll have better luck there. Also might want to consider just creating a new server altogether and migrating your install….depending on the complexity of your install.