It's obvious that Eri is not accustomed to using her quirk. While she is still young, Eri's lack of training and inexperience is her major weakness. Most of the characters also don't have a clearer idea or information about Rewind.  This is why she is unable to control and use her quirk to its full potential. Just to recall, she did not know how to control her power leading to accidentally erasing her father. Training Eri would also be difficult since her powers only affect living beings.