When working in Microsoft Project Professional, you add a Domain user that does not have access to the SharePoint site you get the following error when trying to sync:
We can’t sync resource ‘Joan Soap’ to the tasks list because the resource does not exist on the SharePoint server. This resource and any other resource that doesn’t exist in SharePoint will remain assigned to the tasks in your project plan.
No record of the user exists on the SharePoint Site, therefore, it cannot sync the user as a resource.
Assign the task in the SharePoint site and sync. This will automatically add the resource to Project, including linking it to Active Directory. Once Project and SharePoint can recognize the resource you can continue to work in Project without receiving the above error.