Cause
open requires the campaign to have ends_at.
What to do
Save the campaign with ends_at before opening it.
application/problem+json
{
"type": "https://masnet.com/developers/errors/ends_at_required",
"title": "Missing end date",
"status": 422,
"detail": "...",
"code": "ends_at_required",
"correlation_id": "..."
}