macOS 10.15.1. Software Update won't pick up upgrades
Make sure you use the combo updater if you are trying to manually update by more than one &quot;point&quot; release - 10.15.6 is the latest - https://support.apple.com/kb/DL2048
For future reference, Google can always find these very easily - just search &quot;combo update 10.15&quot; etc &amp; the latest will be the first result.
The combo updater is capable of jumping from any point release to the latest.
It may at the same time fix your Software Update control panel issue, though you may not find that out for sure until the next update.
If that still doesn't work, double check your free drive space. Use Disk Utility &amp; the &quot;Free&quot; value rather than the &quot;Available&quot; value.