What is this, where do I find it in the buy or build mode, and how do I delete it? I've tried the eyedropper, I've tried the sledgehammer, I've tried the wall tool, I've even nuked it with the yellow nuke box but it still remains. Thanks in advance.
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Save the lot first in case something goes wrong!
Boolprop constrainfloorelevation false
Then- draw a square of deck on the square right next to the trellis wall. Then delete the deck with the sledgehammer.
Use the level terrain tool on the upper floor to drag over the area and level the tops of the walls.
When all looks right,
Boolprop constrainfloorelevation true

It worked.....thanks bunches Simsample.

No problem,this is just a wall made with the deck walls- you can either achieve this by using some building tricks, or do like I do and unlock the wall in your game files. You have to treat it like a deck wall to remove it though, so if you don't realise what it is it can be a bit puzzling!