custom_indicator_not_available
What happened
Those three cases get the same answer on purpose, 403, so the reply never tells you whether someone else's id exists. A run that was queued can also end with this code when an indicator it pins is no longer available to the account.
What to do
Use a version of your own indicator that is marked runnable. A version becomes runnable when it passes its checks.
Example
json
{
"error": {
"code": "custom_indicator_not_available",
"message": "One or more custom indicators in this run are not available to your account.",
"detail": {
"request_id": "3c6f9a52-8e1d-4b7a-9f20-6d41a8c5e7b3"
}
}
}