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.