You will see this issue if you have just added a new warehouse/inventory storage location or added a new fulfillment service that is assigned to be fulfilled for a product SKU. Resolving this is very simple. The only thing you need to do is follow steps 1-3 also shown in the screenshot below.
Click on the "Settings" link at the top of the page
Click on the "Fulfillments" link in the side menu
Click on the "Import Locations" button
Now you can try fulfilling the item again and it should work. If you still run into issues. Please do reach out to us via the live chat link in the app or email us at [email protected]