How do I stop my iMac from automatically shutting down after not using for sometime?

1) For starters you should update to the latest macOS version.

see > Update macOS on Mac - Apple Support

2) The latest macOS versions are having various issues with automatic computer sleep mode.

As a test:

Turn On "Prevent automatic sleeping when display is off" and only allowing the display to turn off.

Then just leave the Mac On with display Off and/or shut down for longer periods of non-use.

Compare your Energy Saver settings

and Lock Screen settings.

If you do not want Start Screen Saver, set to Never.

If you do not want to enter the password, set to Never.

There are three different Sleep functions.

1. Display Off aka Display Sleep.
2. Display Off aka Display Sleep w/ Lock Screen
3. Computer Auto Sleep mode or Manual Sleep from the Menu.

If you do not want Lock Screen after Display Off aka Display Sleep, then you need to disable the Lock Screen setting.

You're welcome.