This usually occurs because the assigned date of the assignment is after the end date. For example, if an assignment is assigned on 5/28/17, the end date cannot be before that date.

To fix this, you'll need to change the assigned date or change the end date to be after the assigned date. To do this, you'll go to the appropriate course >
  1. Assignments
  2. Edit that assignment (if the date is in the Future, you'll need to check off Future under the Date View)
  3. Edit Settings and change the dates