Can Polymorph be cast on a Familiar? [closed]

What a creature can be polymorphed into depends on the creature’s level or CR, of which a familiar has neither. Unless you use the CR of the form the familiar has taken. Or, as a spirit that takes the form of a creature, is a familiar a shapechanger and therefore not a valid target for the spell? If you can polymorph a familiar, is it still unable to attack?