Structure spawning

So, I made a small structure that i want to spawn fairly commonly around my custom dimension however on the default settings it spawns everywhere including on top of the the water and in biomes I don't want. I thought that would be a simple fix by changing the "spawn block type" and "biome to spawn in" settings but as soon as i touch either it no longer spawns anywhere what so ever. I thought maybe it was restricting it from spawning in those locations instead of saying it could only spawn there so i flipped it around but still no luck. Anyone have any idea what i'm doing wrong and / or how to fix it?