When I try to install ARKit supported app in iPhone 6 its saying requires the “arkit” capability which is not supported by your's iPhone but then how the demo app of ARKit provided by apple is running on the same iPhone.
I had a similar question, although the purpose was different. I hope this answer helps someone:
If you have an iPhone that came out before the iPhone 6S it will not work. However, you can have an app that has ARKit if you remove UIRequiredDeviceCapabilities from the plist. This way you are able to run (although you will not see anything). This allows you to have an app that has ARKit as a bonus (and only enable/disable a button if ARKit can be used)