You got the Art Dealer's safe combination for Slackjaw, but robbed the safe first
At the beginning of mission 3, across the way from Granny Rag’s Apartment will be a thug beckoning you. Talk to him and then enter the Dunwall Distillery and find a man named Slackjaw. He will give you a mission to retrieve a note and then a second mission where you are required to extract a safe’s code from an NPC.
Make your way to the Golden Cat and torture Art Dealer Bunting until he gives you the code (this is also the non-lethal way of dealing with the target, so simply stab him if you’re going for high chaos). With the code, make your way back to the main district and locate the Art Dealer’s House. It is opposite the building you entered for Slackjaw’s first mission, but is located on the second story. Blink your way up the ducts and then onto the balcony, and finally enter the Art Dealer’s House.
Go straight ahead and then upstairs to a room containing the safe. Dispatch the two guards and enter the code into the safe (the code is random), loot whatever you want, then return to the Distillery and speak to Slackjaw. Upon giving him the safe’s code the trophy will unlock.
It is also worth noting that the Art Dealer’s House contains three Sokolov paintings.
To see the location of the three paintings in the house, please see the video below:
I Got the combo from the Art Dealer, Unlocked the safe, and took all the contents inside but I got no achievement. Do you have to accept a mission from slackjaw before you rob the safe or what? I don't get it.