IPhone – Need help using AirDrop between iPhone 8 Plus and MacBook Air

Your mac (and mine too) is not compatible with airdrop between mac and iOS

System Requirements
To see if your Mac works with AirDrop, make sure you’re in the Finder by clicking the desktop (the background area of your screen), or by clicking the Finder icon in the Dock.
Then, check to see if AirDrop is listed as an option in the Go menu.
If you don't see AirDrop listed, your Mac doesn't support this feature.
In order to transfer files between a Mac and and an iPhone, iPad or iPod touch

• your iOS device needs to include a lightning connector
• your iOS device needs iOS 7 or later installed
• your Mac needs to be a 2012 or later model with OS X Yosemite installed
• Your Mac and iOS device both need bluetooth and Wi-Fi turned on. You do not have to be connected to a specific Wi-Fi network

source:
http://support.apple.com/en-is/HT203106

I unfortunately have no solution to offer you, but I need to point out that the answer by TheBro21 is actually incorrect. AirDrop should work between Macs and iOS devices provided that:

• The iOS device supports AirDrop and runs iOS 8 or later
• The Mac was made in 2012 or later and runs OS X Yosemite

Since both requirements are fulfilled in your case, it should work. I does for me. If it doesn't, I'd keep looking or take it up with Apple (since your iPad Air is still under warranty, I'm sure they'd help you out).

Source: https://support.apple.com/kb/PH18947