Cannot switch on Airplay

The minimum system requirements for AirPlay are:

• An Apple TV (2nd or 3rd generation) with software update v5.0 or later.
• And one of these Macs, running OS X 10.8 (Mountain Lion): iMac (Mid 2011 or newer) Mac mini (Mid 2011 or newer) MacBook Air (Mid 2011 or newer) MacBook Pro (Early 2011 or newer)

Does your setup meet these minimums?

BishBosh65 wrote:

I don't have Apple TV.

I have a 2012 iMac running OS X El Capitan 10.11.6

This is what I have when I click on the Airplay icon. there is no "On" option when you click the AirPlay Display chooser

Okay, in order to make AirPlay work, you MUST have an AppleTV, an AppleTV 2, 3 or 4. Otherwise your computer is working perfectly. You may benefit by reading:

Use AirPlay to display video from your Mac on an HDTV - Apple Support