There is a common issue causing this. Firstly if you are trying to test an order that was created before the app was installed then this won't work. Just create a new order and return that and it will work first time 😀
Sometimes you may find you need to leave the order for 5 minutes before processing the test return to ensure Sorted Returns has received the order details that have been generated.
If you continue to have an issue please get in contact and we will assist.