iMacs not working in Target Display Mode

[rboerner]

Originally Posted by Tomsjotun

Is there any other way to solve it?

As I said previously, Apple has very specific requirements in order for Target Display Mode to work as they describe here --> https://support.apple.com/en-us/HT204592

A key element that jumps out from Apple's documentation is:

"The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed."

This appears to indicate that support for target display mode was dropped in macOS 10.15 Catalina.

If that information doesn't help, I would suggest contacting Apple directly for further assistance.