Before adding notes of calculated type (to make averages of other notes), you must create a type of notes «calculated«.
For this go to menu -> Academic Settings -> Note Type:
Click on the button 
to add a new note type:
- note type: Choose "calculated«.
- Name: Give the note type a name.
- Minimum and maximum: select the allowed range for your note.
- Approved: select the minimum to be considered approved.
- Decimals and rounding: Select the number of decimal places to display and the type of rounding.
Now you can do calculations of final averages in the main notes screen.