While いけません is, strictly speaking, the negative potential form of 行く, it typically means “must not do” - “Because you have a cough, Sunao-san, you mustn’t allow it to get worse”
Bocchi is so shy that Kai-chan is Bocchi’s only friend when they’re in elementary school. Now Kai-chan and Bocchi are going to different middle schools, so Kai-chan wants Bocchi to make more friends so that she’s not alone at school. I feel like requiring her to befriend the whole class is setting the bar rather high, though.