Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
Hi Vernon,
Thanks for posting in the community. We are happy to help you.
According to the error message, if you are the end user, please report the situation to your admin/ IT department.
If you are the admin/IT department, please try the following steps, then check again.
a. Please unassign the Project plan from this user's account via the Microsoft 365 admin center. Later, please wait for a few minutes.
b. Please re-assign the Project plan to this user's account. Later, please wait for a few minutes, then ask the user to test again.
If the situation still happens after trying the above steps, please ask your admin/IT department to raise a support ticket via Microsoft 365 Admin Center> Support> New service request. That support team there will have the correct channel and resources to help you investigate more and find what exactly the reason has caused this situation.
Appreciate your understanding and patience.
Sincerely,
George | Microsoft Community Moderator