It looks like your solution can be found in the comments section of the quest you linked.
Specifically this comment:
If you have accepted but not yet completed
It’s Never Easy, Magni Bronzebeard won’t be in the chamber so you won’t be able to turn this in. You must finish up the It’s Never Easy quest chain, then you can turn this quest in once Magni returns to the Heart.
It looks like your character is currently on It’s Never Easy, so follow through and finish that quest chain.