Black plane in rendering

[baris1 (baris)]

why is the top plane of the object is dark?

[Terry_Chappell]

Maybe the surface normals are missing or bad?

[baris1 (baris)]

what is that suppose to be?
here is the original file
rhino goldbar.3dm (785.3 KB)

[jeremy5 (Jeremy)]

You need to have an environment set for reflections in Render Properties:

HTH
Jeremy

p.s. It was only possible to home in on this after you posted your file. Always worth posting up front!

[baris1 (baris)]

thank you