Cause
The channel is not among the channels granted to the integration. A nonexistent channel responds the same way.
What to do
Use a channel from GET /communications/channels or ask the administrator to grant it.
application/problem+json
{
"type": "https://masnet.com/developers/errors/channel_not_granted",
"title": "Channel not granted",
"status": 403,
"detail": "...",
"code": "channel_not_granted",
"correlation_id": "..."
}