So I have a 2019 13-inch MacBook Pro and 2019, 27-inch 5k iMac. I am attempting to use the iMac as a secondary display for my MacBook with an Apple Thunderbolt 3 (USB-C) cable, but I can't get it working. I've tried activating Target Display Mode via Command-F2, but nothing happens. Any help would be appreciated!
iMac Line (2012 and Later)
A 2019 iMac will not work that way. Only a narrow range of iMac sub-modless support what you want, Target Display Mode (TDM). Support for TDM ceased with the Late 2014 iMacs.
Apple's support document: Use your iMac as a display - Apple Support
Unfortunately, Retina displays do not support Target Dispay Mode. You may still accomplish what you want with various screen sharing methods including Luma Display.
https://support.apple.com/guide/mac-help/use-an-imac-as-a-display-mh30822/mac
As Alan mentioned it simply will not work and no there are no work arounds. If you want an external display for the MBP, you need to buy one that fits your needs.
iMac as secondary display not work
