Operating system installation went wrong and the computer now goes to the panic screen and doesn't start up.
Any tips (with potentially using an external Macbook and or USB drive) how I can repair this Mac's OS and/or use it as an external monitor as it is I think a 2014 Mac?
iMac 21.5″, OS X 10.11
There is no practical way to use that iMac as an external monitor.
The iMac (21.5-inch, Mid 2014) does support Target Display Mode – and is the last Mac to do so. However, Apple added restrictions to Target Display Mode such that no current, or even recent, Mac supports using any iMac as a Target Display. You'd need $80+ in specialized Thunderbolt adapter and cabling hardware to make a Thunderbolt data connection, just to find out that while a Thunderbolt connection is necessary, it is not sufficient.
See the bad news here: Use your iMac as a display with target display mode - Apple Support
That iMac is also too old to be an AirPlay Receiver for AirPlay to Mac, a lower-quality way of using a Mac's screen as a second display.
Given that you can get some decent, if basic, 27" 2560x1440 and 3840x2160 monitors for $300 – $400, I can't see spending much on third-party workarounds (that may involve various compromises) to reuse the 1920x1080 screen.
panic(cpu 1 caller 0xffffff800bb960)
