I have a child flow and trying to use Concurrency, it tells me I need Async in my response, but it will not save when I change the setting to async in the respond to powerapps of flow. Why won't it save the setting?

I have a child flow and trying to use Concurrency, it tells me I need Async in my response, but it will not save when I change the setting to async in the respond to powerapps of flow. Why won't it save the setting?