Unable to use Underlight Angler with the Artifact Forge
Common ProblemsFinished the achievement and the quests to get Underlight Angler, but I can't upgrade it
You must visit the artifact forge in your class hall to purchase traits
The correct forge for this artifact weapon, is located at the Dalaran Fountain.
If you are unable to interact with the Luminous Pearl in Dalaran please equip your Underlight Angler and log out and log back in and you should be able to interact with the Luminous Pearl.
Alternatively leaving the zone and coming back, or Hearthing and coming back while having the Underlight Angler equipped should also let you interact with the fishing artifact forge.
Another thing that you can try is check your quest logs for any pending Artifact quests. If you do, complete these quests and check the Pearl again to see if there is any change.
You can find more information regarding Legion fishing in these two guides: