Take care of the color of the square. Manual page 19:
The color of the outer contour of a square shows the possibility of placement and digging trenches: gray any unit can be placed and a trench can be dug, orange - any unit can be placed, but no trench can be dug, and red - only infantry units can be placed (without heavy weapons). If a square is not marked, no unit of any type can be placed in this location.