Can I un install Ventura? What operating system can I go back to?

Sally1952 wrote:

Can I un install Ventura? What operating system can I go back to?

Resolving issues going forward is always preferred, not backwards.

You are showing 10.13 is that what you want...

You have to erase/reformat/initalize the parent drive as new from Internet Recovery

there are options here—How to reinstall macOS

Recovery (both M1 and Intel) — How to reinstall macOS - Apple Support

You then restore your user form a backup— not with out risk.

You can always make a bootable USB installer—Create a bootable installer for macOS

The older macOS installer backups on this server:

How to download macOS

How to download macOS - Apple Support

You also need to know that you can not install an operating system that is earlier than the one that the Mac came with.