i cant figure out why my greenhouse isnt valid

[Frenbee]

Hi, ive made a greenhouse sort of similar to this before and that one worked fine, including paths and slabs. im not sure why this isnt working, any help would be appreciated

[Frenbee]

i figured it out, the problem was the two line of slabs connecting to the center log beam, and the left log beam. i just moved them down so that they were flush with the bottom of the blocks. i feel like this lay out should work, maybe something for the devs to consider, because it feels a little too limiting. 

[Dilan Rona]

I would say the high walls is not helping much either, but if it is valid, then I guess it works.