Confusion in the naming_series list of options

edit: The forum editor is placing 3 dots where there are supposed to be 2.

Field: naming_series - document: purchase order
Entry a: .DD.
Entry b: PUR-ORD-.YY…MM…DD.-.###
Entry c: PUR-ORD-.YY…MM…###
Response: “Default value for naming_series must be in the list of options.”

Other entries like PUR-ORD-.YY…MM.-.### are working. The above examples are also working for other documents like sales order.

ERPNext: v14.0.1
Frappe Framework: v14.4.0