How to merge two faces to one?

I am quite new at SketchUp and learned a lot from the many instruction videos, but I cannot figure this one out:

How can I merge these two faces to one?

This is the actual Sketchup file:
tmp.skp (14.6 KB)

show hidden gemoetry and delete the line.

The corners of your shape are not coplanar; they can’t be covered by a single SketchUp face because faces are always planar.

That way you can merge the two faces:

But it’s easier if you draw the rectangle from scratch.

And you could also do right click on the axes and choose “Reset”.

Thanks Capottolo,
Unfortunately that does not work…

That makes sence Steve, I will look into it.

Thanks Mihai,
That was very clear. I know I wouldne quicker re-drawing the geometry, but this is meant only as an example.
You made it very clear what is going on here. Thanks again.