One or more items cannot be created. Each entry's field begins with items[n], naming the item that failed; for validation and systemActivityTypeRestricted it is exactly items[n] and nothing more.
Error codes:
validation — the activity breaks a rule this endpoint enforces, or its activity type is not one this company has. message gives the reason, one entry per problem.
activityTypeArchived — the item's activityTypeId names an archived activity type.
recordReferenceNotFound — a record this request points at does not exist. field names that record's id.
recordTypeNotSupported — an activity cannot attach to a record of this type. field names that record's type.
recordAccessDenied — the caller may not read the item's primary record, or one of its related records.
systemActivityTypeRestricted — only JobNimbus itself creates activities of this type.
The batch is all or nothing: when any item fails, this endpoint creates no activity at all.