Beginners often extend their elbows outward while rowing and pull their shoulders up. This causes the body to tense up quickly. The correct way is to put your elbows in and bring your shoulder blades together behind your back with the pulling motion.
Falling into a slight hunchback is another common rowing ergometer mistake and causes back pain. As a beginner, you should make a conscious effort to keep your upper body straight. In order to be able to internalise the technique well, it helps to train with low resistances at first and to increase gradually later.
You should also avoid powerful snatches and sudden releases. Instead, perform the movements fluidly and move the rowing bar powerfully towards the body and back again without letting go completely.