Is Apple airplay available for iPhone 8?

Yes. It is a built-in option. See: Use AirPlay to stream video or mirror the screen of your iPhone or iPad - Apple Support

What are you trying to AirPlay too exactly? It needs to have AirPlay 2 built in. If for example you're trying to AirPlay to an older TV, which isn't AirPlay 2 capable, you won't be able to AirPlay to that TV.