Target display mode

The now completely deprecated Target Display Mode only worked with a limited subset of iMacs acting as an external monitors, and the Macs sending the video signal to the iMac were also constrained by model. We don't know your models.

Understand that Apple have walked away from the technology so you will be largely on your own here.

No one here can answer without the details of BOTH computers, iMac and source. For each computer, do "About this Mac.." and post sub-model and screen size for the iMac, and the model, sub-model of the computer that will feed the signal to the iMac.

iMac:

typical source Mac:

These are roughly the officially supported screen sizes and support model years for TDM:

VIDEO SOURCE COMPUTER (the one sending the video signal to an external monitor):

• A 2019 or older Intel Mac running macOS 10.15 Catalina or older.
• Apple Silicon Macs are not supported†
• mac OS11 Big Sur is not supported.†
• non-Apple device were never officially supported

† — unofficial sources show developing information that some 2020 Macs running can do TDM but only to a 2009 or 2010 iMac 27, not newer models. gain not official.

TARGET COMPUTER (the one to be an external monitor):

• An iMac 27-inch (Late 2009 and Mid 2010)
• any iMac from Mid 2011 to Mid 2014.
• The target computer must run no higher macOS than OS 10.13.X High Sierra.

So we are stuck in your particular situation until you help us help you by providing more details of the two computers involved.

Which Macs are you trying to connect to each other? What are their operating system versions? The mac in Target Display mode has to be on High Sierra. Also Target Display mode is no longer supported in the latest macs. I'm currently connecting 2012 27" iMac with a 2020 27" iMac with Duet Display. It is usable. I would prefer to be able to connect them via TDM but I haven't been able to get it to work.

I hope this helps.

Paul