From what I can see, you're trying to place large lot over sidewalks which you can't, just like you can't put them on streets.
To edit world first make shore to check sizes of lots already placed, then scan around for objects and stuff, you can "scan" it with small size lot just to see what areas are free (some areas are marked as nonroutable even if they look free). Then plan what you want to replace with what. Joria mastered world editing in game so she will be the best to share you some tips.