[New Builder] Why is this block unbuildable?

After a fairly successful town I decided to ramp up the difficulty challenges in buildings. A single block in an easily reachable position gives them trouble. I’ve built ladders to it both sides. I’ve reloaded. I’ve hit R. I’ve turned building jobs on and off. I’ve paused the building. The red block remains. Here’s the save.
1527004696997.zip (9.2 MB)