1) Find the relevant transaction, and check if you have data entered in Ship and Invoice tabs. Ship and Invoice tabs can be left Unauthorized, but it is required that you have Shipping information and Invoice lines in the said tabs.
2) Check if you have a complete and valid shipping address in the order, and also make sure that this address in the transaction is present and matching in the addresses listed under the customer set up, if not in the customer set up, you will need to create the address in the customer set up, matching the shipping address in the sale.
3) Also, when you're setting up the Shipstation integration, please make sure that the Log in and Password you use in Shipstation belong to a User in DEAR (not ShipStation). This user needs to only have access to a Single DEAR Organisation only, as otherwise you will get 503 error in ShipStation.