What type of underlayment is appropriate for metal roofing?
You need to always follow the metal roofing manufacturer’s recommendations for installation, but generally you can use 1-layer of 15# building paper if the slope is 4:12 or greater and it’s standing seam metal roof with concealed fasteners. 

You need a minimum of 2-layers 15# building paper if it’s a slope between 2:12 and 4:12. 

Below 2:12 slope you need to follow the manufacturer’s recommendation for installation. (They’ll probably recommend a peel-and-stick underlayment.)