You received an Empty Segment error when testing or sending a campaign but you know there are Contacts in the Segment.
The most common cause of this error is due to an account setting called "cross-post merging".
The cross-post merging setting evaluates if recipients in a segment (or segments) have already received the same campaign within the last 24 hours.
The feature determines duplicates by checking the from line, HMTL content, and segment. If they're the same as a previous recent campaign, it will prevent the campaign from going out.
This setting is designed to protect you from sending to contacts who are in more than 1 segment, or sending the same campaign to the same recipients more than once. You may see a campaign in your Analytics with no tracking recorded if a campaign was hit with cross post merging.