Not a new question; I see many others with same failure for Target Display Mode
There's plenty of discussion on this topic. Just as others, I've followed all steps to use mid-2011 27" iMac in
Target Display Mode to 'No Joy' results.
I did get, at least, some keyboard recognition by 'matching keyboard settings' especially 'Use F1,F2, etc. keys as standard function keys. That settings change got recognition, tone sound, as I pressed 'Fn key to use the special features printed on each key. But, no result other than the tone, so I eliminated that Function Key choice as standard functions keys.
I am using the, many years used, USB Keyboard on the iMac
I am using the, many years used, BlueTooth Keyboard for my new 2018 Intel Mac Mini
Using FlowChart & Scientific Method to retrace each step, I've followed every option discussed in the Target Display Mode article. I'm still failing to implement Target Display Mode.
So, I understand there are plenty of frustrated users who dream of keeping their old 'qualified' iMac as a Great Display Monitor.
I'll give it a rest for today; perhaps back to the task tomorrow.
Thanks for the chance to investigate and discuss this 'popular iMac TDM' subject.
iMac 27″, macOS 10.13