From Gamer Heroes: “In order to find the Lucky Coin, you must go into the sewer that connects to the first area. The worlds are randomly generated, so I cant lead you directly to it. Search the corners of the sewers while you are making your way to the subway to find it. When you finally have the coin, return to the base and then go speak with Ace. She will thank you and give you a nice Magnum in return to the item. The magnum does a lot more damage, but you lose out on ammo capacity. Be sure to equip a mod to it if you are switching.”
Source: N4G PC Where To Turn In The Lucky Coin In Remnant: From The Ashes