My intel iMac does not get into the login screen. It displays a black screen with an error message. I tried several time starting in safe mode but it did not work. What other options do I have next to reinstalling the OS?

Try booting into Safe Mode since Safe Mode prevents third party software from launching automatically during boot & login.

Make sure to also disconnect all external devices in case one of them is causing a problem.

If this doesn't work, then try booting into Internet Recovery Mode using Command + Option + R and running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If so, then you will need to perform a clean install of macOS which will destroy all data on the drive.

You can also try running the Apple Diagnostics to see if any hardware issues are detected.

It also really helps to know the exact model of the iMac along with the version of macOS being used. You can get the exact model by entering the system serial number here:

Check Your Service and Support Coverage - Apple Support