。
It's shameful not to keep a promise.
Literal
Promise [obj-を] not-keep-[nominalizer-のは] shame [copula-だ].
[Negative verb]+のは[noun]だ is a clean way to predicate a judgment on an action. 恥 ('shame, disgrace') is a culturally loaded concept in Japan — the shame-based moral framework has deeper cultural roots than English 'shame,' tied to concepts like 恥をかく ('be shamed') and 恥ずかしい ('embarrassed, ashamed').