I want to iPhone 6 version 11

you may need to clarify what you are asking.

if you are asking about running iOS 11 on and iPhone 6.. if you're running iOS 12, you cannot downgrade the iOS

if you are asking about upgrading from an iPhone 6 to an iPhone 11, Apple no longer sells the iPhone 11, you'd need to find one somewhere else, such as your cellular provider.

No.

you can’t downgrade iOS version

As I commented.. you cannot downgrade your iOS. So if you are trying to run an app that is not supported with newer iOS version, you will not be able to use that app