Airplay

Streaming from iPhone or iPad to what? To Apple TV box? If so, which model?

Identify your Apple TV model - Apple Support

Find your Apple TV model number - Apple Support

What is its version system software?

Does AirPlay work for other apps?

Have you tried the convenience of the YouTube app on Apple TV instead? (No AirPlay needed.)

Use AirPlay to stream video or mirror the screen of your iPhone or iPad - Apple Support

• Have you tired the in-app casting abilities of the YouTube app? That includes AirPlay.