Target display not working

I am trying to connect iMac (21.5 inch, 2019; Big Sur 11.2.2) to iMac (21.5 inch, late 2015; El Capitan 10.11.6) to use the latter as a second monitor. I am using Thunderbolt cable and tried Target Display Mode (command +F2), but nothing is happening. I also bought a new thunderbolt cable and tried, but again it did not work. I previously set up Target Display Mode on multiple iMacs before, and troubleshooting tips (restarting the computers) usually solved any issues, but no this time. Any one please help!