Thanks for the report.
I checked the code. The message you encountered can happen if you lack the keys ITMKeyDungeon2, ITMKeyDungeon3
["ITMKeyDungeon2", "Lower Dungeons Key", "The", "key", "A rusted key, which provides access to the deeper dungeons.", None, 500, ["no sell", "unique", "no find"],{}],
["ITMKeyDungeon3", "The Pit Key", "The", "key", "A rusted key, which provides access to the deepest reaches of the dungeons.", None, 1000, ["no sell", "unique", "no find"],{}],
ITMKeyDungeon3 is an endgame item for the Goblin King ending, so it might be that one? That can be opbtained from Gwynn if the Goblin King Quest is unlocked.