Social platforms expire access tokens periodically (or when you change your password or revoke access). When that happens, the channel is flagged as needs reconnect and its posts will fail until you fix it.
To reconnect:
- Go to Channels.
- Find the channel marked needs reconnect / expired.
- Click Reconnect and re-authorize on the platform.
After reconnecting, retry any failed posts from the Calendar or post's detail view. If a channel keeps disconnecting, revoke BulkPublish in the platform's app settings and connect it fresh.