The spell conjure volley has one piece of ammunition or one thrown weapon as its material component. The spell effect further describe the requirement of the said ammunition or thrown weapon: they must be nonmagical.
You fire a piece of nonmagical ammunition from a ranged weapon or throw a nonmagical weapon into the air and choose a point within range. Hundreds of duplicates of the ammunition or weapon fall in a volley from above and then disappear.
What happens if I use magical ammunition or magical thrown weapon? Do the spell fail and the spell slot consumed? Do the spell just don't get cast at all and I'm like an idiot throwing a magical axe into the air waiting for something to happen?