Elihu asks this question to emphasize that Job cannot do this. AT: "You cannot spread out the sky ... a mirror of cast metal." (See: [[rc://en/ta/man/translate/figs-rquestion]])
In biblical days, mirrors were made of metal. Elihu speaks of the sky giving no rain as if it were as hard as solid metal. (See: [[rc://en/ta/man/translate/figs-metaphor]])