Lander Hello there,
This situation might have happened because of two potential reasons:
-You have clicked "Fulfill items" on Shopify > Orders page. You do not need to do this on Shopify, as Oberlo will automatically do this for you later
-You have an auto-fulfillment setting enabled in Shopify. If this is a reason, in order to fix it, go to Shopify > Settings > Checkout. Under the "Order processing" section select "Do not automatically fulfill any of the order's line items." Disable automatic fulfillment, and things should work fine with all your future orders.
As for this particular order, firstly, you need to cancel its fulfillment through Shopify. The orange "Order Products" button will appear in Oberlo, therefore, secondly, click on it to purchase an item from AliExpress supplier. If you are not sure how to do this, check out this guide: http://help.oberlo.com/orders/how-to-purchase-items-from-suppliers/how-to-order-aliexpress-products-using-oberlo