I would guess that somebody killed that boss while you were respawning and coming back to where you spawned it. Especially if you summoned the mob last week and returned now (if I understood correctly?).
You can purchase the Dank Scroll in the item shop. Press "I" in-game, it should be in the right column on the shop menu that pops up. You'll need AP to buy it, but you can use AP earned from offers to buy the Dank Scroll.
Since this mob is tied with a quest, you should try giving up the quest and returning to whoever gave you the quest. Take the quest again, they'll give you the spawn item again. Not sure by what you mean by not being able to do any other quests, can you clarify?