Rules on the column in the table is not the way to implement your requirement.
1. Make a separate table of the Roles with RoleID and RoleName
2. Add a RoleID field to the employee table. Set its default to null. If Role is required, make the field required.
3. In the relationship window of the BE database, create a relationship between the two tables and check the enforce RI box.
4. On your data entry form, add a combo that uses the Role table as a control source. If you let the wizard build the combo, it will hide the RoleID field and only display the text.