If you have selected both Service and Is Required checkboxes on the project policy page for a project, the Service field is enabled and required for the project, and empty fields are not allowed.
When you create a work plan for the project:
If you already specified a default service in the project's Project Details tab, the Service field for the root task inherits the default service setting;
If you have not specified a default service in the Project Details tab, you will be prompted to specify a default service.
When you click Create blank work plan from the Summary tab of the Project Overview page, the Choose Default Service dialog opens.
To specify a default service, click the selector icon, select a service from the list, then click Create to proceed.
When you click Create work plan from a template, the Create Work Plan from Template dialog opens.
In addition to specifying a work plan template, you also need to specify a default service for the work plan.
When you click Create work plan from another project, the Create Work Plan from Another Project dialog opens.
In addition to specifying a project, you need to specify a default service for the work plan.
Each new task item you add to the work plan follows certain service inheritance rules. See Service Inheritance Behavior.
After you finish adding task items, you may edit the Service field value for each of the tasks or summary tasks individually. You can also specify different services for different task items on the same work plan. See Setting Services for Tasks.