I have explained that in several treads, the reason for that is that your wall thickness doesn't match your extrusion width and the slicer just leave the space blank.
A 0.4 nozzle usually extrudes a 0.48 width filament, in your case looks like 4 perimeters compose the wall so to seal it your wall needs to be 0.48 X 4 = 1.92 mm, anything bigger than that and you will see the space, anything smaller and it will reduce it to 3 perimeters and you will also see the space.
To solve your problem either adjust your wall thickness to match your extrusion width or adjust your extrusion width to match your wall thickness.