Add or set up a Group by Box to allow sorting by Category:
- Right-click the word Categories in the task list column header (with column names or Arrange By).
- Click Group by Box if it is not already highlighted.
- Drag and drop the Categories column header into the new Group By Box above.
- If more than one Group By Box appears, drag the other one down to the column header and drop it there.
- (You can click the Group By Box to reverse the grouping order.)