For the summoning test, you do not need a mushroom directly. You trade the Quetzalcellona Mushroom for a Kohar Root by talking to the imp Nixfix. If you have done that already, try checking your inventory and dropping any other items called 'root' and all runes other than the Wisdom rune. Does the hero have all of the components (Feynur potion, spider leg, Kohar Root and Wisdom rune)?
