It’s hard to say what you are missing without seeing what you’ve done but in the video, the author draws an edge on the roof and then draws a vertical line at the top end of that first edge. He’s drawing the side of the chimney. For you, the fix may be as simple as orbiting your view so you can get that vertical line in the blue direction.
If your view approximates to an axis a rectangle defaults to that axial plane.
Or simply draw some lines vertically - using axis-lock shortcuts like shift or the arrow-keys…
One thing to keep in mind is that working in SketchUp is more like sculpture than painting. You need to work around the model and change your view to suit the part you’re working on at the moment.
The four keys on your keyboard with up/down/left/right arrows on them - usually to the right of the main keys.
You can tap on these in some SketchUp tools to lock the drawing inference, axes etc.
Holding the Shift kwy down also locks the line you are drawing to its current colored axis or parallel/perpendicular to an edge you’ve hovered over etc…
Google some help, there’s lots…