No upper skin created on Luban

I designed a simple model (text only) in tinkercad and imported it as STL to Luban (snapmaker 2)
No matter what kind of printing I choose it generates no upper skin (or little upper skin not on whole of the part) as shown in the image:

what is the reason for that ?

It looks correct. Skin (yellow) is inside the walls (green and red). Skin doesn’t cover walls, if that’s what you’re expecting.

I expect the yellow (skin) to cover all in order that the upper layer to be snooth and look good. And I expect that the inner wall (green) wont be seen at all

Nope. That’s not how it works.

@lightov I believe orcaslicer (maybe some other slicers too?)has the functionality to have only 1 wall on top surface while using more walls on rest of the model…

Did you try less walls? Try a wall size of maybe 0.4mm-0.5mm and a infill of 85% (not 100% to minimize infill artefacts).
Then it should work like you expect.