Select any Four Store
job in either Crime.net or Crime.net Offline, preferably any easy 1-2 star job available. The gold bar is a random spawn. If it spawns, it will be in one of the safes located inside one of the many stores. Once the mission starts, put on your mask, and look for safes. They're usually behind counters but can also appear in back rooms. Safes take 3 minutes to drill though without any skills, can be cracked in 45 seconds with safe cracking, or C4'd in 10-30 seconds depending on how much C4 is needed.
The achievement will unlock for the whole party as soon as the safe opens if it contains a gold bar. If none of the safes contain a gold bar, proceed to quit out or complete the heist, and try again.