You need the treasure table in the mod's .pak file. Unpacking any mod that adds items into the game, looking at item modding tutorials and the bg3 wiki's "how to code an item" section will tell you this, and how to do it. Just make sure your I_ItemName matches the name you gave the item in RootTemplates and the game should find it. Unless your code isn't correct in which case keep checking UUIDs, file paths and names etc until it shows up.
And speaking of coding, make sure your dummy_root is named correctly in the visual bank! It needs to match the dummy_root the armature has in blender, so unless you specifically named it dummy_root, it's not gonna be called that.
If you experience any issues, I recommend hopping over to the down by the river discord and posting in the modders helpboard there since they have a big modding community and someone there will know how to help you troubleshoot.
