Self-explanatory, I cannot build the multiblock structure due to a single error. I also make sure its 3x3x7. The controller said that the block in the middle and at y lever 1 of the current structure must be air, that's.......weird so I remove it, but it says the block that I just removed cannot be air , must be the casing instead but then if I remove it goes the same.
It makes me inaccessible to the later fuel upgrades because of this. And here are the images: