在如下循环中使用类型转换将List<dynamic>转换为List<String>：

class ErrorMessage {
  /// {@macro error_message}
  const ErrorMessage({ 
    required this.statusCode,
    required this.message,
    required this.error,
  });

    /// Creates a ErrorMessage from Json map
  factory ErrorMessage.fromJson(Map<String, dynamic> json) =>ErrorMessage(
      statusCode: json['statusCode'] as int,
      message:
          (json['message'] as List<dynamic>).map((e) => e as String).toList(),
      error: json['error'] as String,
    );

  final int statusCode;
  final List<String> message;
  final String error;
}

尝试以下型号

class Response {
  Response({
      this.statusCode,
      this.message,
      this.error,
  });

  int? statusCode;
  List<String>? message;
  String? error;

  factory Response.fromJson(Map<String, dynamic> json) => Response(
      statusCode: json["statusCode"],
      message: json["message"] == null ? [] : List<String>.from(json["message"]!.map((x) => x)),
      error: json["error"],
  );
  
}

您需要分别对String的List执行map操作，如下所示：
变更

factory ErrorMessage.fromJson(Map<String, dynamic> json) => ErrorMessage(
    json['statusCode'],
    json['message'],
    json['error']
  );

到

factory ErrorMessage.fromJson(Map<String, dynamic> json) =>ErrorMessage(
      statusCode: json['statusCode'] as int,
      message:(json['message']).map((e) => e as String).toList(), //👈 Emphasis on this
      error: json['error'] as String,
    );