https://www.freecadweb.org/wiki/Basic_P ... torial_017
Within the section "Feature with pocket and external geometry" at the point "To create pocket which has the sloped rectangle as it´s outlet .....". In the next paragraph beginning "Select New Sketch .." there is no instruction to create the rectangle within that new sketch. There is mention of the "sketched rectangle of the slope" and then "Then you should see the rectangle inside the 3D View." But that last mentioned rectangle is the sloping one, and we haven't created the rectangle in the sketch.
The step should now say something like:
Create a new rectangle within the external geometry.
Also within that paragraph the instruction is to use the spacebar to make things visible/hidden. I did not see any change when I used the spacebar (I think visibility implies a visible change!). I had to scroll a long way down the View section of Sketch003 to find the Visibility property and make sure it stated Visibility: true.
I am using macOS 10.14 Mojave with FreeCAD Version: 0.18.16117.