。
She can't possibly be serious.
Literal
Serious [attributive-の] [expectation-はず] [subject-が] not-exist [feminine-わ].
~はずがない ('cannot possibly / no way') is the negative of ~はずだ ('should be / must be'), expressing strong denial of a possibility. Literally 'the expectation (of X) does not exist.' 本気 ('serious, in earnest') is a common noun/な-adjective. Subject is omitted, but the feminine わ and the dismissive sentiment suggest a female speaker reacting skeptically to someone's claim.