Question about tilemap

I have a problem with tilemap. Picture will best explain my problem.

where the red arrows are pointing. How do i get rid of this???

Make that part transparent so you can see the bricks behind it?

You can't place tiles over other tiles in one tilemap. Each tile can only be placed on each cell. Your image is completely fine, that's a normal behavior.

Make each "wall" as a separate tilemap object or draw your tiles differently to accommodate all possible solutions - like part of the wall overlapping different part of the wall.