After an update to Lightroom for desktop, you cannot sync photos with Lightroom mobile and are not logged in to Lightroom mobile account.
To restore sync with Lightroom mobile, sign in from the Activity Center. For details, see Set up Lightroom for sync.
Alternatively, you can also log in to Lightroom mobile account by choosing Edit > Preferences > Lightroom mobile (Windows) or Lightroom > Preferences > Lightroom mobile (Mac OS). For details, see Lightroom for mobile preferences.