iMac 17” screen shutting down

Try and see if you can boot the Mac in the safe mode to see if the issue persists

If it does not persist in the safe mode, it is caused by an external piece of software, you might want to evaluate the software installed to your Mac

Start up your Mac with Apple silicon in safe mode

1. On your Mac, choose Apple menu  > Shut Down.
2. Wait for your Mac to shut down completely. A Mac is completely shut down when the screen is black and any lights (including in the Touch Bar) are off.
3. Press and hold the power button on your Mac until “Loading startup options” appears.
4. Select a volume.
5. Press and hold the Shift key, then click Continue in Safe Mode.
6. The computer restarts automatically. When the login window appears, you should see “Safe Boot” in the menu bar.

Start up your Intel-based Mac in safe mode

1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.

If the issue does persist in the safe mode, it is most likely a hardware issue, please Contact - Official Apple Support

to set up hardware service for your device.

Apple has not made a 17” iMac in about 20 years. Are you sure that is what you own?????