I attempted to create naturally spawning structures using mine craft Add-ons. I used feature rules and structure features and a .structure file from the structure block. When loading the behavior pack in game it gives the error that no structure could be found or it has no blocks which it does have blocks. The way to recreate it is to download Ore_Mod.zip and extract it into the behavior pack folder in com.mojang. Then create a new world and enable experimental features.  Also other add-ons that are supposed to generate naturally generated structures don't work either.