Try to build wall and it shows error (construction failed) I still haven't reached my maximum amount..

Tap the castle, tap layout then it will reorganising the map and the walls.

Tap on your castle then tap change layout then tap apply layout

The bug is related to previous wall placements that aren't registering as removed properly.

Either the wall moved, or was a demolished wall or gate location. You then move your roads and structures into that space. Everything looks normal at that time, However you start getting wall limits of not being able to build. I found when I moved everything out of that area, closed the game, reloaded, magically walls would appear in that open space that were not supposed to be there. I was then able to move them elsewhere and fix my wall limit count.