This achievement is for gathering 2000 oil. During the tutorial, you already send your army away once to gather oil. I got around 1300 oil from that gather. To gather more oil, go to the map and find one of the areas with a red oil barrel next to it. Click on it and select "gather". Then send your army away to gather oil. You don't have to wait out the entire time for it to come back. I left my army there for 10-15 minutes and then called it back. That already resulted in gathering 7600 oil (depends on army size). Just have a look at how much oil is in there to gather. Once that has dropped by at least the amount you still need to get to 2000, call the army back to get your achievement once they arrive.