What happened to anyone who could not keep up with the march in book night?

The book you are referring to appears to be "Night" by Elie Wiesel. In this book, anyone who could not keep up with the marches during the Holocaust was often left behind and faced extreme danger or even death. The marches were grueling and occurred in harsh weather conditions, and the weak, sick, or elderly often struggled to keep pace. Those who fell behind risked being shot by the Nazi guards or left to die alone in the freezing cold. The SS guards had little mercy for those who could not keep up, viewing them as expendable and unimportant.