Two major reasons behind failed orders are stated below-

1. Product not Found - When an order is placed for a product that is only available on SHEIN or does not exist in our app, the error "Product not found" appears in SHEIN orders. As a result, the order cannot be successfully synchronized to the Shopify store and displays the problem. If the product is available on both SHEIN and Shopify, you must link it with the appropriate Shopify product in order to fix the error and sync the order to Shopify. If the product is not yet available on Shopify, you must first create it there, import it into the app, and then link it with the appropriate SHEIN listing. 

2. Unable to Reserve Inventory- You must go to your Shopify store settings in order to correct the mistake with your orders. Locate the "Locations" setting, where you can see all the Shopify locations, and make sure the "Fulfill Online Orders from this Location" checkbox is selected for that particular location. Make sure you have merchandise available for that particular place if permission has already been set for the location.