The error message "Inactive Customer Account" shows up when saving an Item Fulfillment using a FedEx shipping item.
This message relates to an issue with FedEx Billing, therefore, we recommend contacting FedEx Customer Service at 1-800-GoFedEx (1-800-463-3339).
If you have multiple FedEx accounts, make sure to provide the Account Number related to the Location similar to where the Item Fulfillment is coming from.
Note: This can only be reproduced on FedEx shipping items that have the Shipping Label Integration enabled.
In addition, an enhancement has been filed to change the pop-up message to something more specific. Users may refer to Issue # 185452 - Transactions > Sales > Fulfill Orders - Provide a friendly error message when Shipping Carrier Account used is inactive instead of "Inactive Customer Account"