Rather late to this, and I doubt you still care, but for some reason the game allowed you to place the marker where some of the constructions you would have to build are beyond the edge of the building area of the map. If you take that save file you posted and zoom in on the right side of the monument, you can see 8 very faint red-tinted column outlines, each one two squares directly to the right of the columns you had already built, and all of them one square beyond the eastern edge of the map building area. In short, this monument is impossible to complete without God mode on, which I used to fill in those spots and complete the quest. Imo, I'd say this is a bug, for the game to have allowed the marker to be placed where it was to begin with when the player could never have actually completed the monument without cheating.
I've split this to a new topic since it's a new bug. Will fix, thanks!
Fixed, thanks.