If there are no oversights in the preparation, success is certain.
Literal
Preparation [on-に] oversight [subject-が] is-not-[if-ば] success [topic-は] certain is.
ぬかりがない ('there is no oversight / no neglect') is a set phrase — ぬかり is from 抜かる ('to be careless / slip up'). 確実 ('certain / sure') is a na-adjective. A formal advice sentence about thorough preparation.