Using 2021 iMac as a monitor

Hi

I am trying to use my late 2012 iMac as a monitor for my 2020 MacBook pro. I have followed the instructions to set this up and used a thunderbolt cable to connect the iMac to the MacBook, then tried using command + F2, as well as fn + command + F2. This did not work. I have tried restarting both machines. The iMac is running OZ X El Capitan.

Are there any other suggestions to get this to work?

Thanks in advance for your assistance!