Using 2011 iMac (OS High Serira 10.13.6) as a monitor for my MacBook Air M2

I have a late 2011 iMac running High Sierra 10.13.6. I have an M2 MacBook Air My iMAC has the old Thunderbolt/ Thuderbolt 2 socket. I bought the Apple Thunderbolt 2 to Thunderbolt 3 adaptor and a Thunderbolt 2 cable, connected them and pressed Command F2 and no joy - rebooted, reconnected as per support pages and still no joy. Same again and tried FN-Command-F2 and nothing. AirPlay on the iMac can see the MacBook. Screen mirroring on the MacBook can see my Roku Stick but nothing else. I am stumped. Any ideas.