I've previously created an issue on GitHub because I was unable to assign owners to a private channel. I encountered the same error response when I executed the request directly in the Graph Explorer without going through the SDK (tried both
- Request's payload: same as this example
404 page not found
The user who executes the request has the appropriate delegated permission required to manage channel membership, as dictated here.
Note that I mitigated this problem by instead defining the channel membership when I created the channel rather than post creation, but I thought it would be good to raise the issue so that it can be resolved for good.