It happens sometimes when Shopify fulfill the order automatically or order is wrongly fulfilled by mistakes.
Disable Shopify Auto-fulfillment Feature
Shopify has a feature to automatically fulfill paid orders. In order to process order in Topdser, please disable it.
Go to Shopify - Setting - Checkout
In the Checkout setting, find Order processing and select "Do not automatically fulfill any of the order's line items" . Don't forget to save the settting!
Cancel Fulfillment for Orders
Go to Shopify order details page, click “more” and select “cancel fulfillment”
Now the order will go back to unfulfilled status in Shopify.
Then go back to Topdser, you will see the order status change to “awaiting order“ status. Now you can process the order.