Iphone6s has taptic engine or not, can I use a public API called
No, the taptic engine currently is only available on the iPhone 7 and 7+.
At this time, only the new Taptic Engine found in the iPhone 7 and iPhone 7 Plus support these APIs. Other devices silently ignore the haptic requests.
On iPhone 6s, your device can call UIFeedbackGenerator, but nothing will happen since the taptic hardware is not present in the phone.