My building has a bubble with a red circle with a line through it.
This is because the building’s is not connected to a path. It could be that there's no path tile at the building's entrance or it could be that there is a path to the building but that path is not connected to the main path.
To fix this you can try one of these:
1. Build a path from this building's entrance to the main path.
2. Move the building to where there's a path
3. Rotate the building if there's a path on the other side of the building
4. If the building appears to be connected to a path but the red circle is still appearing overhead, it's possible that a path piece was accidentally deleted somewhere along the way. Go into edit paths mode and have a look along the path to see if you can see a gap. If you do, add a path piece to fill in the gap.