It is made of 2 planes, which you can see from a top view (the white 2cm above the blue):

I just drag lines from its vertices, but the non parallel side is not created… Is this by design? Is there anything I can do about it?
(thanks in advance)

That is because the two skewed cuts do not lie in the same plane. SketchUp faces must be planar. You can verify this by drawing a diagonal across the area. It forms two triangular faces because any three points always lie on the same plane.

You can draw the diagonal and then hide it to create the illusion of the shape, but you can’t make a single planar face span a non-planar boundary. That is not a SketchUp limitation, it is basic geometry.

Actually, this wasn’t indeed the shape I needed. I don’t know what I was thinking, but tt was evident, as you pointed out that this couldn’t be any similar to plane face. Yet it is good to remember this trick for close to plane cases that don’t really need any accuracy at all…