When typing barely hard(not gently and slow) keyboard blinks and phone vibrates once.this never happened on my other iphone 6 plus.
This is an iphone 6s plus.and it doesnt happen on all keys,it doesnt happen on emoticon or numbers(123)key
Did you already try the general troubleshooting steps?
1. Switch off your phone and restart your device.
2. Force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose data.
3. In case this does not work, set it back to factory settings, without using the latest backup. Test the phone. If it works, restore from the backup.
Use iTunes to restore your iOS device to factory settings - Apple Support
4. If factory resetting does not help, get the phone checked by Apple:
Keyboard blinks ,vibrates and doesn't type when typing fastand barely hard
