You can not use your induction hypothesis because H simplifies to n + n = m + m while IHn expects n + n = S m + S m. In other words, the argument m you want to feed the induction hypothesis changed, which is a symptom that you need to generalize over m (with revert m) before doing induction n (actually you can just not introduce m and H before doing induction n). Besides, you can also destruct m instead of doing an induction on it, the induction hypothesis IHm is useless.