have to use password instead of finger print, why?

It isn't clear to me what you mean by "open." You should be able to use the finger print to unlock it when you raise the lid to wake it from sleep mode but if it has been shut down (which I very rarely do) then a password is required at startup.

See the following from Use Touch ID on your Mac - Apple Support

Lift the lid of your MacBook Pro or MacBook Air to turn it on, or press Touch ID. Every time you start up, restart, or log out, you need to type your password to log in to your user account. After setting up Touch ID, as long as you're logged in, you can use your fingerprint instead of typing when you're asked for your password.