Buy or Get app in App Store without fingerprint ID - Help!

A friend has an iPad and long ago tried setting up fingerprint ID. Now He wants to get a new app but is struggling. First, the prompt implies to press your thumb IN THE CENTER OF THE SCREEN - THE DIALOG BOX instead of using the HOME BUTTON. We are past that user interface bungle now, but the fingerprint is not working.

How can he get to typing a passcode?