。
You shouldn't break your promises.
Literal
Promise [obj-を] break-[should-べき]-[neg-ではありません].
The negative form of ~べき: ~べきではない ('should not'). Polite ~べきではありません with です adds a formal register. Slightly more formal than the alternative phrasing ~のは良くない ("it's not good to"), which carries a similar meaning in a softer, more conversational register.