The value you are attempting to delete has probably been selected from this field by a user in the system. Drop-down values that are in use cannot be deleted. However, they can be disabled. To disable a value, when editing the user defined field, ‘on the User Defined Field Settings for… ‘page, select the value and click the Disable button. The value will be saved for users who have already selected it, but will not be available for selection by other users.