Q: AirDrop from iOS 9 to El Capitan not working

Switch off your bluetooth and delete this file:

/Library/Preferences/com.apple.Bluetooth.plist

When you switch your bluetooth back on it should work.

Which MacBook Air do you have?
It must be late 2010 or newer.

-> https://support.apple.com/en-us/HT201862

From the following document at Use AirDrop to send content from your Mac - Apple Support

AirDrop system requirements

To send items to an iPhone, iPad, or iPod touch, or to receive items from those devices, you need a 2012 or later Mac model with OS X Yosemite or later, excluding the Mac Pro (Mid 2012).

To send items to another Mac, you need:

• MacBook Pro (Late 2008) or later, excluding the MacBook Pro (17-inch, Late 2008)
• MacBook Air (Late 2010) or later
• MacBook (Late 2008) or later, excluding the white MacBook (Late 2008)
• iMac (Early 2009) or later
• Mac Mini (Mid 2010) or later
• Mac Pro (Early 2009 with AirPort Extreme card, or Mid 2010)

Learn how to identify your model of MacBook, MacBook Pro, MacBook Air, iMac, Mac mini, or Mac Pro.