This FAQ covers this error specifically:

All order items must have status Pending or Ready To Ship  

This error may occur when Omnivore is attempting to push tracking information to the Iconic but the marketplace cannot accept the tracking data and returns the error.

The order in the iconic has to be a particular status or state in order to be able to receive the tracking.

If something in the order has changed, such as a line item was cancelled, this will result in a mis-match in the order state and this error can occur.

Fix and Solutions

The first thing is to make sure that you are receiving the failed order alerts that Omnivore sends to sellers.  Go to Settings -  Account Summary - Contact - Opt In.  Check default alerts and notifications and add yourself in to get specific alerts.

If they are in place then you will receive a notification when or if this happens on an order.

Check the status of the order in Seller Centre for the iconic, you will most likely confirm that some kind of change has happened to the order or an item in the order.

Because we do not poll for order updates aside from accurate tracking updates, in the case of this error you will need to manually set the order to shipped in the iconic, set to manually fulfilled in Omnivore and manually set the order to shipped/fulfilled in your eCommerce Store.