Never mind, I'm just a massive idiot.
I've been running the "minecraft_server.1.12.2" instead of "forge-1.12.2-188.8.131.5255"
In fact, I can't even place any block associated with the mod.
Does anyone know why ores such as "Copper and Tin" won't spawn in my server?
It works fine on single player.
I also can't place the ore blocks in my world either even though, I can put them in my hotbar and search for them
I'm also running on 1.12.2