She shouldn't go out alone at night.

Literal

She [topic-は] night [to-に] alone [with-で] should-not-walk.

出歩くべきではない — 出歩く (であるく, 'to go out walking, to go around') is a compound verb. べきではない is the negative of べき ('should'), expressing strong advice. ひとりで ('alone') marks the manner.