Rey insists on going since she's the only one who can get there. Leia initially tells her, "No," but then relents. She's cautious about letting Rey go because she knows of her natural anger and how easily she could be corrupted and fall to the Dark Side. Similarly, Yoda and Obi-Wan didn't want Luke to head to Bespin - he wasn't ready and was rash. They feared Vader would turn him. However, Leia realizes she has no other choice, so she lets Rey go with her blessing.