turning on imac but a question mark flashing in the screen
The first owner erase all setting. So when I turn it on a question is flashing on the screen
Question marked as
If your Mac starts up to a question mark - Apple Support
Turn off your Mac
Start up from macOS Recovery
To start up from macOS Recovery, press the power button to turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R.
Select Disk Utility from the list in macOS Recovery, then click Continue and follow the steps to repair your startup disk with Disk Utility. These steps also describe what to do if Disk Utility can't repair your startup disk, or Disk Utility doesn't see your startup disk.
When done, restart your Mac: choose Apple menu > Restart.
If the question mark returns after restarting your Mac, repeat steps 1 and 2 to turn off your Mac and start up again from macOS Recovery. Then follow the steps to reinstall macOS.