This is most likely due to no grade plan grade or transfer grades being included from that year. You will need to add a grade plan to the grade plan tab or mark either Include Transfer grades or Include Term Transfer grades in order to save the year to the Grade Average. 

Steps to save new year to Grade Average: 
  1. Academics > Grades > Grading Setup > Calculations
  2. Edit into the Grade Average
  3. Select Step 2 School & Years
  4. Click +Add Another Year
  5. From the Select a school year drop down, select a school year
  6. From the Select a school level drop down, choose a school level 
  7. Mark Include transfer grades if applicable
  8. Select Step 3 Grade Plans
  9. Click the down arrow for one of the terms for the school year added in step 2
  10. Choose the grade plans as needed
  11. Mark Include term transfer grades if needed
  12. Click Save & Close