Sorry for the late reply, I usually check here everyday, but I completely forgot about it for a few days. Are you using an older version of the game for some reason? At one point, there was a bug where the variable used to see if you have a quest to hand in or not was bugged for the end of the 5th dungeon quest, but that should have been fixed a while ago, and there's even a backup event in town to try and fix this bug, so the only way I can see you still getting it is to have finished the quest a while ago. Regardless, it should be fixed with your save, on file 21. I just changed the variable and walked into the guild to make sure the hand-in option was there. Thanks for letting me know about that.
Mega