Airdrop between iPhone and iMac

What model year is the iMac?

For Airdropping to work between iOs devices and Macs the Mac needs to be a 2012 model or later.

What happens when you try to Airdrop? does the Mac not appear in the iPhone's Airdrop options?

Does the iPhone not appear in the Mac's Aridrop section in finder?

See here: Use AirDrop on your Mac - Apple Support

Well, both have to have WiFi and Bluetooth enabled. Both have to be logged into the same Apple ID. They have to be within approx. 30ft of each other. You could try restarting your Mac and iPhone.