I think you told me in game that you're watching a YouTube video to help with this quest. I assume it's accurate, but this walkthrough may also be of help if you missed a step. https://forum.dmgamestudio.com/viewtopic.php?f=4&t=17901 If you don't have a quest to talk to that particular guy, then...

Unforunately, that is an area that player can't get to. So, that chest will go unopened this time. Better luck next time!