how to repair a startup volume on IMac 2019?

Hi crocket49,

You need to follow these steps:

1. Shut down the Mac.
2. Turn on the Mac, and immediately press and hold Option + Command + R until you see a spinning globe.
3. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner.
4. Go to Disk Utility.
5. Select "View" in the toolbar —> "Show All Devices".
6. Select your main hard drive or SSD. This will be the top, non indented hard drive. 
7. Click Erase, and input the following exactly: (If you cannot select erase on the main hard drive, erase the "Container" or “Macintosh HD”.)
8. Name: Macintosh HD
9. Format: APFS
10. Scheme: GUID Partition Map (if shown)
11. Click Erase. If “Erase Volume Group” is shown, use that instead. 
12. Quit Disk Utility
13. Go to the reinstall macOS app.
14. Follow the prompts, and install on Macintosh HD.

Also read: What to do before you sell, give away, or trade in your Mac - Apple Support.

Cheers,

Jack

What exactly do you mean repair? If you suspect the hard disk is in trouble, please restart in the Recovery Partition (hold down Command + R on startup) open, Disk Utility and run First Aid several times. Look for any errors, if any errors appear that means the HD should be replaced.