Complex sentence: The two stages of the Energia launcher functioned as designed, but due to a software error in its attitude control system, Polyus' orbital insertion motor failed to inject the payload into orbit.
Split into simpler sentences: The two stages of the Energia launcher functioned as designed, but the Polyus payload failed to reach orbit. Due to a software error in its attitude control system, the burn of the Polyus' orbital insertion motor failed to insert the payload into orbit.