Sometimes it is impossible to place various items flush with any side of a wall or tower.
Sometimes it works fine, others not so much.
Playing on PC via GeForceNow.
Video example here: https://imgur.com/79y4Ziw
Be sure to unmute sound in player.
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That’s very strange, they’re glowing green so they should be placeable and you were well under the build limit. Not sure what’s wrong there.
There are several bugs related to base building (including possible permanent game corruption when you place walls at the extreme edge of base building zones - DO NOT TRY unless you have backed up your savegame).
I’ve encountered many situations where items couldn’t be placed in a certain order, even if the tiles are green. My solution has so far been to tear down the existing wall, and place things in the “reverse” order - or just try some other order. Sometimes items/walls can only be placed in one order, but not the reverse.
Somehow, I’ve always managed to build what I wanted by shuffling the order in which I build things. 
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Thanks I will give that a try.