I think its because you are adding the full RGB channel to the metallic node, the node only need a 0-1 value, but by connecting the white connector to metallic, you actually give it 3 values (1,1,1). Try only connect one of the color channels to the node.
EDIT: Btw, how does the preview look like in the material editor? If the material is black on the sphere/cube, then it’s the material that has the issue and not your model. At least you can rule that out. But try test the one channel above.