The African Union hasn't done anything about the crimes being committed in Burundi since the crisis began. It has had observers there since the beginning, but I don't believe they've produced any reports on the human rights violations. As far as I know, they aren't permitted to visit secret locations or meet with national intelligence service officers to record arrests and arbitrary detentions. I don't think the African Union has the power or the will to investigate the human rights violations.
On November 6th, 2018. See this statement in context.