A leave of absence rule does not operate as a validation until it is associated with a specific leave type on the Validations/Supporting Data tab. The administrator must first add the rule to the leave type and save the leave type. This establishes the relationship required for Workday to configure a validation message in the context of that leave type. The administrator can then use the leave type's Related Actions to configure the message.
By default, Workday can display the leave of absence rule's Description as the warning or error text. A separately configured validation message can provide more precise guidance and may include static text, report-field values, or calculated-field values relevant to the request. This allows the same underlying rule to be associated with multiple leave types while supporting context-specific messaging.
The message is not selected from a fixed list of predefined options. Configuring it directly on an unassociated rule would not provide the leave-type context required for the validation. Accessing Related Actions before adding and saving the rule would likewise leave no saved validation relationship to configure. The correct sequence is to associate the rule, save the leave type, and then configure the message through Related Actions.
Reference: Workday Education - Absence for Administrators: Leave of Absence, "Validation Errors and Warnings" and "How to Associate a Leave of Absence Rule"