I need help finding an issue in my build

[vernal plank]

I'm practicing a design that I've made multiple times and for some reason it's not letting me place walls in a specific spot because there's not enough space

[tropic spade]

I'll be holding classes this weekend to teach people if you want I can try to make some time to spare to help

[vernal plank]

I know how to build I've just never had this issue with this base I've built it multiple times I thinking maybe it's just the terrain or something

[tropic spade]

if your trying to make a gap cap it with a triangle make it with a full circle till a triangle is placed facing the other cap it with a square delete all triangles build back with squares repeat the process 3-5 times depending on how large you want the gap