This is because the Task has been disabled. To enable the Task, navigate to
  1. From the menu in the top left select Core
  2. Navigate to Security > Roles
  3. Search for the All School Role
  4. Click into the All School Role Link
  5. Click Tasks
  6. Click Edit and enable the Task titled "My Groups - Master Page"​  and select both checkboxes
  7. Click Save & Exit