IPhone won't connect to iMac

I'm trying to connect my iPhone to my iMac both have just been restarted after current updates. They will see each other and I will set a prayer request with a code which I accepted but then the iPhone says that my iMac is not supported. i am running 12.5 on the mac and 15.6 on the phone. I was trying to attempt an airdrop....

Thanks,

Rob