Are you using freehand tool on a drawing tablet? I’ve been trying to work on the same goal, so I can make 3D reliefs of drawings.
If you select your edges and go to scale tool and see a cube, the edges are not 2D/coplanar. This happens a lot if using x-Ray view/style. Instead, draw directly onto the reference image or onto a grouped rectangle.
It helps if you go to Model Info and turn Length Snapping > off. Even then, face generation is very spotty. Doesn’t seem to be any solution to face generation fails except to scrap your drawn edges and redraw them until they finally click. *edit: or, for a more mathematically structured illustration such as your example, use arc and line tool rather than freehand