To log in with your user in the POS App, you need a four-digit PIN, which you can set in your Shopware Admin.
You can access the user settings for the Admin Account at the bottom left via Your Profile. In the General tab, in the Pickware Authentication section, you can set the app PIN.
You can find the settings for all other users under Settings → System → Users & Permissions (Benutzer & Rechte). Click on the relevant user there and set the app PIN in the Pickware Authentication section. You can find more information about user roles for POS here.