When fulfilling orders, Shopify first creates a Fulfillment Order, which is used to fulfill the line items associated with your order.  Each Fulfillment Order has a field associated with it called supported_actions.  This tells EZ Fulfill what can be done on the Fulfillment Order. 

Some actions include move, hold, request_fulfillment, and create_fulfillment, amongst others.  

Before fulfilling your order, we must check if the create_fulfillment action is found in the supported_actions field on the fulfillment order we're trying to fulfill.  If the create_fulfillment action is not found, EZ Fulfill will return an error letting you know your order can not be fulfilled.  Shopify does not allow Fulfillment Orders to be fulfilled if the create_fulfillment action is not found in the supported_actions field.

At this point, you'll have to take action on the Fulfillment Order outside of EZ Fulfill to ensure the Fulfillment Order can be moved into a fulfilled state.