Cause
The modality (identified or confidential) is not in allowed_modalities of the grant.
What to do
Use a granted modality.
application/problem+json
{
"type": "https://masnet.com/developers/errors/modality_not_granted",
"title": "Modality not granted",
"status": 403,
"detail": "...",
"code": "modality_not_granted",
"correlation_id": "..."
}