How to make BiometricPrompt non-cancelable?

BiometricPrompt does not allow that. So you won't be able to make the system-provided biometric prompt non-cancelable. But you can detect whenever user cancels the dialog.

So an option would be, to show again the biometric prompt after user cancel it (which I think would be a bad user experience) or use alternate user authentication:

override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
        if (errorCode == BiometricConstants.ERROR_USER_CANCELED) {
            // User canceled the operation

            // you can either show the dialog again here

            // or use alternate authentication (e.g. a password) - recommended way
        }
    }

check it out

if (Build.VERSION.SDK_INT < Build.VERSION_CODES.P) {
    supportFragmentManager.fragments.forEach {
        if(it is DialogFragment) {
            it.dialog?.setCanceledOnTouchOutside(false)
        }
    }
}

There are some devices that still have this issue. An work around will be to get root view and add an overlay view with clickable method set to false.

    ViewGroup  viewGroup =  ((ViewGroup) yourActivity.findViewById(android.R.id.content)).getChildAt(0);

    //create your view
    Display display = mActivity.getWindowManager().getDefaultDisplay();
    Point size = new Point();
    display.getSize(size);
    View view = new View(yourActivity);
    view.setId(R.id.overlay_view);
    view.setLayoutParams(new ViewGroup.LayoutParams(size.x, size.y));
    view.setBackgroundColor(ContextCompat.getColor(yourActivity, R.color.black));
    view.setOnClickListener(v -> {
        //do nothing prevent click under this overlay
    });

    //add your view on top of the screen
    viewGroup.addView(view);

    //call your biometric dialog
    ....

    //on callbacks even if it is error or success call remove view
    viewGroup.removeView(view);

You have to use the version 1.0.0-beta01 or later.

Now it is the default behavior:
Touches outside no longer cancel authentication. Back button cancel authentication still.

You can see the changelog:

Changed behavior to not allow BiometricPrompt to be cancelled by a touch event outside the prompt.

You can check also the rewiew report.
No new API.