More than likely this is because there are 2 Milestones with the Same Status. You cannot have the same Status for 2 Milestones unless one is Status change of no change. To change this navigate to:
  1. onBoard> Settings > Checklist
  2. Click Edit next to the Checklist Title
  3. Click Edit next to the Milestone for the Decision
  4. Also check the  Milestone before and after it and if the same status, edit one of them to be Status Change "No Change"