Can't place large planting bed on my roof??

For some reason I am unable to place a large planting bed on my Obsidian 2 story house's roof? There is nothing currently there as I just purchased it. It let's me place potted plants etc there but not this or the Aether dragon statue. no idea why unless there is a size issue. There is plenty of room for the items but they just won't let me place them. Very frustrating as I bought this house specifically for the ability to garden on the roof.

 

@vavavoom, if this is a row house, on a row lot, i do not believe you have the external item count available to place it. a planting bed takes an item slot for each planting spot, so the large ones take 20.