Head to the distillery office to find Nicki. After the cutscene, open your map and look for the green mission marker in the Bayou area. Like the lieutenant missions from earlier, you have to head there and steal something from the Dixie Mafia then deliver them to one of Burke's rackets to increase the total earn. You should do two runs to fully increase Burke’s earn at this point of the game. Once done, the game will also notify you that the earn for all rackets for the Burke's operations is already at max; you can do more of these runs when you assign more rackets to the Burke.