@Grape1207 apologies for the delayed response - I just noticed we didn't respond to your question.
Chalk is currently not compatible with your iPhone 6+. However, we are targeting a 2.1 release in July that will enable support for iPhone 5s and above.
We keep this download page updated to reflect the current "verified" devices: https://chalk.vuforia.com/download-chalk
I'll let you know when the 2.1 release ships and I'd like for you to give it a try!