After resolving your Send to groups and subtracting Don't send to, nobody was left. Common causes:

• the selected list is empty or all its members have Can Receive Email: No;
• the segment genuinely has no one right now (e.g. Delivery tomorrow when there are no orders for tomorrow, or Pickup on X Day with a Trigger Date that has no pickups);
• a value-based segment needs its extra field: fill Trigger Date, Trigger Compare Value or Trigger Checkouts Count;
• your exclusions remove everyone (e.g. Send to All Users and Don't send to All Users);
• the customers' email addresses are empty or invalid, or the customers are suspended.

Use View Recipients on the Review step to check the count before pressing Send Campaign.