Thanks for reporting this.
This is because there were trade carts walking across the foundation. While units are on top of a foundation the structure cannot be built.
I would guess that the second issue would be the minimum distance required between civ centres. Once I deleted the foundation you had placed I could place a new one.
I have modified the tickets title and description to give a more accurate description. Priority has been lowered because this has an easy workaround of manually stopped the flow of trade units.