Unable to reinstall OS X on an old Mac

Hi,

It may be your iMac boot into Recovery Mode.

Following article could help your issue.

Mac startup key combinations – Apple Support (AU)

I just wonder if your iMac connects Internet via WiFi or Ethernet. If it connects, you can download and install macOS via the net.

And if you have another Mac, you can create bootable macOS installer on USB memory.

Create a bootable installer for macOS - Apple Support

Hi,

Not sure how old of your iMac, though if it's a 2017 model, macOS Ventura is the latest which is available at Mac App Store or you can upgrade from System Preferences > Software update.