Shopify added a new feature called "Request Fulfillment" for the orders. While it may useful in few cases, it creates some orders errors and make you unable to place the orders on DSers.
To avoid facing this issue, DO NOT CLICK "Request Fulfillment" on your order on Shopify.
How to fix it
In case you have accidentally clicked on "Request Fulfillment" on a new order on Shopify.

The order will still be in "Awaiting order" tab on DSers but you won't be able to place the order.
A message will appear on top of the order.

You need to go to Shopify, and click on the order. You will see the status as "In Progress".
You need to click on the "More" button, and select "Mark as fulfilled".

The order will be fulfilled on Shopify and DSers, but you still need to actually place it.

To do so, go to the Fulfilled tab on DSers, select the order, then click on the "More Action" button and select "Rollback order".

The order will be unfulfilled on Shopify and DSers.

You can now place the order normally.
Don't hesitate to contact us if you have any questions!