Why doesn’t target display mode work?

After reading online my iMac should work as a second screen for my new macbook. I have a thunderbolt to usb-c adapter and have been pressing command+f2. Below are the specs of my iMac, my understanding was that it wouldn’t work for late 2014 onwards, so why isn’t this working for me please?

iMac (21.5 inch Mid 2014).

1.4 GHz Dual core Intel Core i5.

8GB 1600 MHz DDR3.

Intel HD graphics 5000 1536 MB.

Thanks.

EDIT

I'm using a Macbook Air 2020 version that is running Catalina 10.15.7. The iMac is running exactly the same OS.

The cables I have are Thunderbolt into Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter.

When I press Command + F2 absolutely nothing happens, no sound, no change on screen at all.

Solution 1:

Macbook's with USB-C ports do not support Thunderbolt 3 and therefore the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter can not be used for Target Display Mode. See Use your iMac as a display with target display mode and About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter

However, if your new MacBook really is a 2020 MacBook Pro, then your question is a duplicate of Target Display Mode not working on MacBook Pro 13" 2020. According to IW1234's answer, Apple Support stated they do not have this feature in newer devices.

I should also mention the 2014 iMac is not suppose to be running a macOS newer that High Sierra. However, I have a 2013 iMac with High Sierra and Catalina installed. When running Catalina, Target Display Mode works for video, but not for sound.

Solution 2:

The solution was that the iMac was running Catalina and that was too new an OS for target display. Have now rolled the iMac back to High Sierra. Thank you