Target Display Mode

I am trying to use an iMac with target display mode but it doesn't work. I have a 2020 iMac 27 with Catalina and a 2013 iMac 27 Late with Hgh Sierra. They are connected with a Thunderbolt cable and a Thunderbolt 3 to Thunderbolt 2 adapter. I have tried using the Command-F2 and Command-Fn-F2 keys on the 2013 iMac and nothing happens. I don't understand, is there something I am forgetting? Please help!