You need a detailed surface scan for that info for some reason.
But the value is system wide - not body specific. So don't worry about it if you know the system to be pristine.
Try again? ¯\_(ツ)_/¯
Or look in the right place on the system map?
It took me several months myself to spot the place where the reserves are noted for the first time.
In the systems tab, top left there's some icons. The second one in gives body details so once you click on system object move the mouse over to the left panel and scroll down. Mind you cant say I've ever seen the word pristine - just metallic, metal rich, icy and rocky.
Have I been doing this mining thing all wrong then - only been at it a couple of weeks![]()