I ran into a problem with completing this quest and I wanted to share how I resolved it in case other people encounter a similar problem.
- First, what others have said about the 'current Mythic cloak' being the wrong item ID remains true. You need to run it on Normal, Heroic, or Mythic 0 (no keystone).
- Second, you need to have unlocked the ability to summon Jani. This is done by completing a few short quests in Dazar'alor and starts with 'Pests' until you finish 'Curse of Jani'. You do not need to have acquired the achievement 'Get Hek'd'.
- Once the above two conditions are met you can go to any of Jani's trashpiles and after interacting with them to summon Jani, you will be able to accept the quest 'I Am The Trashmaster'
I did all of these things. But I couldn't get Jani to appear.
I worked through the common issues mentioned in the comments and elsewhere online but my active Loa was 'Kimbul' and not Jani. I tried equipping it (or leaving it in my bags). Walking into the city (instead of flying). And, of course, logging out and in intermittently.
Eventually, I learned that the reason I couldn't get Jani to appear was because I had completed the quest to unlock adding Jani's trashpile as a statue (place of worship) in Dazar'alor. This made Jani's trashpiles function like a scrapyard for me. When I picked one of the other two Loa's instead (I chose Gonk) and returned to the nearby Trashpile in Dazar'alor. I was able to interact with it and Jani appeared for me and allowed me to proceed.
Chronicler Ash'tari, at 50:35 (right near the stairs) is who you speak with to change which Loa is represented in these locations. One for Zuldazar, one for Nazmir, and one for Vol'dun. You need to select the option for Zuldazar and pick either Gronk or Pa'ku.
I hope this helps anyone else that might be getting tripped up on this quest like I did.