Cause
The channel_id differs from the one of the already created publication. The channel does not change.
What to do
Send the original channel or create another publication with another external_id.
application/problem+json
{
"type": "https://masnet.com/developers/errors/immutable_channel",
"title": "Immutable channel",
"status": 409,
"detail": "...",
"code": "immutable_channel",
"correlation_id": "..."
}