Target Display Mode -- Pressing command-F2 does nothing
Solution 1:
iMac 27 5K Late 2014 model and later no longer support Target Display Mode.
Solution 2:
I have found that Target Display Mode will work, but only if I do command-f2 from the login screen. After logging in, the command does nothing.
Hope this helps someone! Running 2011 27-inch iMac as display with MacBook Air and a thunderbolt cable on El Capitan.