airplay displays off - no option to turn on

I am trying to set up the ability to connect to airplay via the top menu bar.

Considering what is written below - what else do I need to do to add the ability to connect to Airplay via an icon in the menu bar?

I am running High Sierra now.

I have gone into system preferences>displays.

I have ticked the box that says 'Show mirroring options in the menu bar when available.

However the drop down menu within system preferences>displays says that Airplay Display is OFF and does not have an option to switch it on.

There is an airplay device on and available.

When I go to System Preferences>sound the existence of an airplay device is confirmed

Following apple support advice / forum answers, i have performed the following already:

1. update both iMac / Apple TV to the latest software version
2. confirmed no device restriction applied by Apple TV
3. confirmed mirror airplay function works well in my other device including my macbook pro / iphone se
4. confirmed subject iMac has switched off all anti virus / firewall in the subject iMAC already
5. confirmed subject iMac and apple tv are connecting the same wifi network with strong wifi signal
6. confirmed subject iMac mirror display option is not available in safe mode / by guest login too
7. tried reinstallation MacOS already (problem exists in Sierra, and remains after updating to High Sierra)

Could you please advise what else i need to do to enable the mirror display in my subject iMac again? Thanks