can I run an app written in Xcode 9 on iOS 12.2?

If your app is already on App Store, (and your app is not using any features removed in iOS 12.2) yes.

If you are developing your app since now, no. Xcode 9 cannot run your app in iOS 12.2.

You should better also know that you cannot submit apps built with Xcode 9 to App Store.

hi, plz help me about this error?

I always have a problem with my simulator (XCode 9)

It always Failed or could not attach!