More than likely the cause of this is because the Assignment/Assessment was not added to the Gradebook (the Add to Gradebook checkbox was not marked) in the Assignment.
To remove this Assignment from the Gradebook:
  1. Click on the Assignments Tab
  2. In the Date View filter on the right, select Previous (if was active for a past date), Active (if active now), or Future (if active for a future date)
  3. Click the red X next to the Assignment Name that you wish to remove
  4. Click Confirm