A customer cannot create user with Office 365 service in POA Customer CP due to the error message "Invalid input 'up_UserPrincipalName' is invalid: Office 365 configuration is not complete. You need to add desired sub-domain in onmicrosoft.com zone to Office 365 application."
Office 365 APS application installation in a customer subscription is not yet completed, in particular, subdomain in the onmicrosoft.com domain was not yet created in the Microsoft Office 365 Cloud. It means that creating customer account is not yet completed on Microsoft side.
Most likely there are failed or pending background tasks in POA Task Manager related to the problem subscription provisioning.
Log into POA Provider Control Panel, find all failed, rescheduled and unprocessed tasks related to the subscription in question in Task Manager, solve problems with tasks (if any) and process them. Use the customer subscription ID to filter all pending tasks in Task Manager.
Also, POA Customer Control Panel shows the ID of the Microsoft Office 365 APS application instance installed in a particular customer subsription at Microsoft Office 365 > General like on the screenshot:
Having the ID of the APS application instance you may find all tasks related to it using the instructions in the Knowledgebase article #115693 APS: How to find POA tasks for particular application instance.