Manage Columns: Add Task duration

Good evening, fantastic tools,
tried to add more columns by opening the Custom Fields Manager, only the first three row are in: name, Begin date and End Date.
Cannot add duration:

  1. Select “Duration” in the list
  2. Click “Show selected”.

Result: the selected columns (Duration in your case) will be shown in the table.

For sure working perfectly, thank you for support, it was too obvious!