Building utilities on a wall corner

The problem is that PA does not recognise entrances placed on the corners of foundations. You need to put the entrance at least one square away from any corner. In your case, although you have built a new building up against an existing wall, PA considers this a separate building and the square you have put the entrance on is a "corner" (even though it won't be once the foundation has been built.

When you want to put an entrance up against another wall this can mean placing an entrance where you don't want it just to get the foundation built; then having to dismantle/brick up the original entrance and add a new entrance where you actually want it.

From my comment above:

There's currently a bug with removing unfinished buildings. Steel beams still appear even after cancelling the build command. Adding a new outdoor floor such as grass or dirt over that spot will clear the construction area.