Air play to LG Smart TV

Why does Apple TV air play and home kit setting come up …

Where does that come up? On you iPad, or on the big screen?

… when connect video to tv?

Is your TV AirPlay-enabled? Or does your TV have a connected streaming device that is AirPlay-enabled?

Is it configured to allow AirPlay receiving (on|off setting)?

Did you add the TV as HomeKit accessory, with a designated named room? Please do.

LG TV support of Apple AirPlay 2 and HomeKit - LG Support

Add a smart home accessory to the Home app - Apple Support

Are both devices connected to the same home network?

What are you trying to stream? Not all content is treated the same.

I never have any problem with my iphone. Only on my ipad.

Hmm, that is odd. I would think both devices would have the same ability for this. Did you try the same content with both devices? Are both devices on the matching version iOS/​iPadOS?

I am trying to stream the Pac12 Network

The Pac-12 Now app preview page says these app editions have the exact same requirements for iPhone and iPad. Another indication that the devices should have parity.

The Pac-12 website doesn’t mention AirPlay, that I could find. Weird.

Perhaps ask the developers for their how-to description. How to contact an app developer - Apple Support

I'm not really interested in another app. Do I have to do the homekit setup?

The message seems to indicate so. The Home app is bundled software with iOS and iPadOS. Adding the TV to your HomeKit configuration should be a one-time setup, then forget about it. I can’t think of any downsides.

Also verify the settings for Apple AirPlay on the TV. Possibly there is a reason there for the iPhone to behave differently from other devices? If unsure, please describe the current settings.