iphone xr cannot use air drop to my imac running High Sierra

I am trying to use Air Drop to upload photos to my iMac. The iMac is running OS High Sierra 10.13.6 and the iPhone is a brand new iPhone XR. The iPhone is connected Bluetooth and recognized by the iMac and both wifi and bluetooth (obviously) turned on. I cannot move files in either direction because the iPhone does not show up on the Airdrop menu and the same is true in reverse. In fact - when I click the share icon on the iMac - nothing happens. Of course I can connect via USB (and I will) but this is one fo the features I ought to be able to use. Can anyone suggest a fix? Thanks in advance.