Do you have a user selected in the Channel?
* If not, please select a user that you have already authenticated with Twitter on the front-end and hit save.
* If you do already have a user selected, I would suggest going into your app at
apps.twitter.com and click the 'Regenerate My Access Token and Token Secret'. After this, re-authenticate on the front-end with that user and then refresh the channel in the backend.
* Probably obvious, but if the channel is not published, select publish and then save it.
Let me know if this helps.
-Melissa