Can't build Gates

[ValEl]

Hello,

i'm trying to build a fence with a gate for the animals. The Fence is no problem but i can't build the gate. It shows always a red square. I tried it on different floors, but it keeps red and can't be build. Also tried to build the gate ON the fence, but it doens'nt work either. Has anyone an Idea what i'm doing wrong?

Greetings

[Senor Smoke]

fence on both sides.
& it has to be a fence, cant put a gate between a fence and a wall