Target Display Mode not working on iMac 2013 running Catalina 10.15.6
I am trying use my iMac 2013 (21.5") in Target Display Mode as an external monitor for my MacBook Pro 2020 but when I press Command-F2 on the iMac, nothing happens. Both machines are running macOS Catalina 10.15.6 and are connected using a Thunderbolt cable and Thunderbolt 3 to Thunderbolt 2 adapter. I am also using the original Mac keyboard on the iMac. I read somewhere that Catalina supports Target Display Mode on an iMac 2013.
Is this correct and if so, why is this not working for me?
MacBook Pro 13″, macOS 10.15