If you selected only the Service checkbox from the project policy for a project, the Service field is enabled but not required. It means that empty fields are allowed.
When you create a work plan for the project (by clicking Create blank work plan, Create work plan from a template... or Create work plan from another project... from the Project Summary tab of the Project Overview page), the Service field for the root task or Service fields for all task items inherit the default service setting if you already specified a default service in the project's Project Details page; otherwise the Service field(s) will be empty. See (Optional) Specify a Default Service for a Project.
Each new task item you add to the work plan follows certain service inheritance rules. See Service Inheritance Behavior. If there is no default service or parent service, the Service field for the new child task will be empty.
However, 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.
Note: Based on your Project Policy settings, all Service fields for task items on a single work plan are either required or non-required. You cannot have both required and non-required fields on one work plan.