What am I doing wrong? Preferences.
Your line is not connected to the vertical line of the box.
If it was you could not dimension a gap.
He draws the line line with a snapped coincident start point. He waits for a blue cross when starting the line, and He doesn’t even mention that important piece in your puzzle.
That means the vertical line from the box exists in his sketch, so you and the presenter have different sets of preferences for sketching, about projecting when referenced.
Might help....
Fusion
