1. Visit a course as an administrator.
2. Create two assignment activities (do not provide an ID number).
3. Click on 'Grades' under 'Course administration'.
4. Under 'Categories and items' select 'Simple view'.
5. Click on 'Add grade item' and fill out the name on the next form and click 'Save changes'.
6. Click on the calculator icon next to the newly created item.
7. Add the calculation "=[[assign1]]+[[assign2]]".
8. Next to the text input fields for these assignments enter assign1 and assign2.
9. Click submit and see you get an 'Invalid formula' message.

Solution:
Enter the ID number for the assignments in the settings then visit this page which will show the ID numbers.