C.O.D. Client-

 

This shows on the shipment screen when a customer is setup with a $0 credit limit in the customer setup screen under the Billing section. 

 

 

You will need to update it under the billing section on the customer setup screen.
It's under Credit Limit- Select 'Adjust Customer Status' to make the changes.

Below is a screenshot for reference-

 

 

Once it's adjusted, reload the customer on the Shipment screen and it will be fixed.