Not able to assign a resource to a task in project worksheet

Saving fails with the validation message "The planned hours of task assignments have exceeded the hours estimated for the project team.", despite having a project team assignment for the same resource and the same start and end dates.

The validation failure also occurs due to other assignments of the same resource in the same project. Find the other assignments of the user in the same project, and compare them with the existing resource requests / project team assignments. All task assignment dates and hours should have a corresponding project team entry (i.e. a resource request with Assigned status), otherwise the new assignment, even if there is a project team entry for it, may trigger this validation error.