I can't find a non-subjective answer to this question. The wiki says:
Hare's Void is called Lover's Bandages. Through her Void, she is able to repair any object or heal any person. When used, the Void wraps itself around the target and begins to glow and reverse any and all damages. The process takes time depending on both the object or person being healed and the extend of the damages.
My hypothesis is that the extent of the damage done to Ayase's legs is beyond Hare's ability to heal/repair. Due to this, Ayase cannot be healed.
On top of this, technically Ayase's legs aren't "damaged". What I mean by this is that they're fully healed in the sense that they don't produce pain. However, when they were healing, she wasn't able to have surgery or proper treatment (or it wasn't possible to treat) to allow her to walk. Thus her legs healed improperly, and thus Hare cannot heal her as there isn't actually anything for Hare to heal.