Lvl 4 weapon quest bug

[Motenai_Ronin]

Could be a # of things. You may not be lvl 70 (in which case the Umbala NPCs won't even let you start the quest), you could have started another Lvl 4 weapon quest by talking to a number of npcs in Nilfheim (in which case that quest has to be completed before you can progress) or you have started a different Umbala weapon quest (in which case that one must be finished).