As the Android Developer docs say
Even if your app doesn’t use accounts, you still need to provide an authenticator component. If you don’t use accounts or server login, the information handled by the authenticator is ignored, so you can provide an authenticator component that contains stub method implementations. You also need to provide a bound Service that allows the sync adapter framework to call the authenticator’s methods.
There is an entire article on Creating a Stub Authenticator. I realise that this question is old and an answer was accepted long ago, but I felt that a recent addition to the official docs should be included here.