I use iphone 6 and the thumb is not activating
I can’t thumb in my iphone 6
Question marked as
Did you replaced the screen? Or repaired by a 3rd party?
Read here: Use Touch ID on iPhone and iPad - Apple Support
and here: Get help with Touch ID on your iPhone or iPad - Apple Support
If no joy, make a genius bar appointment with Apple for an evaluation.
They have tools to test your Touch ID.