Issue: When a user is trying to create or modify a Purchase Order in Microsoft Dynamics 365 Business Central the user has a validation error that states "You cannot have more than one user plan assigned to each user".
Cause: The reason for this error is that Microsoft Invoice Office 365 application, available with Office 365 Business Premium licensing is built on Business Central. There are considerations when implementing Business Central if the Microsoft Invoicing application has been enabled. For additional information for using both applications please review https://docs.microsoft.com/en-ca/dynamics365/business-central/about-reuse-company-invoicing
Solution: This error occurs when a user has both license for Microsoft Invoicing AND Business Central license selected for their user. These are mutually exclusive. Ensure that that if Business Central license is On that the Microsoft Invoicing is Off.