When you add element, you may need to select other elements from the model. In this case, the Command-Manager may open with selection-boxes.
You MUST select the elements that are contextual to each selection-box, in order, from top to bottom, before you can add the new element.
1.Click an element in the graphic-area or the Assembly-Tree that is contextual to the top selection-box
The next selection-box becomes active automatically.
2.Click an element that is contextual to the next selection-box
3.... continue to click elements that are contextual to each selection box, from top to bottom, in order.
When all selection-boxes have elements, the button at the bottom of the Command-Manager becomes active (colorized)
4.Click to complete the command.
See more: Command-Manager