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You shouldn't rank jobs as high or low, you know~.
Literal
Occupation [dative-に] noble-lowly [object-を] attach-[must-not-ちゃいけない][assertive-よ](elongated-~).
A gentle moral admonition. 職業に貴賎(の別)なし is a famous Japanese saying ('there is no high and low of occupations' — all work has dignity); this sentence is an active imperative version of the same sentiment. 貴賎 ('high and low, noble and base') is a classical/formal binary. ~ちゃいけない is a casual contraction of ~てはいけない ('must not X'). The elongated ~よ~ gives a gentle, drawn-out moral-lesson tone.