Hmmm if you have done these objectives;

Speak with Bibimu outside The Sunken Temple of Qarn.
Use the Duty Finder to enter the Sunken Temple of Qarn.
Deliver the wardstone to Slafborn.

It could be a few things.

Ever since a few updates past you have to be the same job that you started the quest with in order finish it.

See if you can just redo the dungeon and clear it to refresh it, ( see if you can drop the item first).

Llast resort abandon the quest and re initiate it. The time you have spent on it you could have finished it over 50 times.