Check that the driver is active and has set a password, and that they type the same email you added them with. On the driver's page, press Resend invite (or Reset password) in the Login details card to email a set-password link or create a temporary password.
A driver who has forgotten their password can tap Forgot password? in the app and follow the emailed link. A driver who already drives for another company on FleetKraft logs in with that same email and password.