Cause
scheduled_at is only accepted with state: published.
What to do
Send state: published along with scheduled_at, or remove scheduled_at.
application/problem+json
{
"type": "https://masnet.com/developers/errors/scheduled_requires_published",
"title": "Scheduling requires publishing",
"status": 422,
"detail": "...",
"code": "scheduled_requires_published",
"correlation_id": "..."
}