1021: Not available.
A declared field or part is not available yet.
HTTP status: 422. May also be reported as a field-level finding in error.details[].
A reserved field or part you supplied has no implementation yet and is refused rather than ignored, so a value you expected to take effect can never quietly fail to. It appears in details[] under a 422 refusal.
Troubleshooting
Section titled “Troubleshooting”- Remove the reserved field or part named by details[].param.