Cannot erase ~500GB from startup disk
This will clear all the data on your drive!

You can erase the whole volume by clicking the View button and choosing Show All Devices. Then select the top-level entry and click Erase. Set it to APFS and GUID and click Erase. Name it Macintosh HD (or whatever you prefer if that's not your preference). Then run the installer on your new Macintosh HD and you'll get a clean install with no data.