how to return list of api response values in flutter

API client: 
class ApiClient extends ServiceAPI {
  final http.Client httpClient;

  ApiClient({
    required this.httpClient,
  });

  Future<Map<String, dynamic>> callApi(Map<String, String> headers, String body,
      String apiname, Method method) async {
    // SmartDialog.showLoading();
    final url = baseUrl + domainPath + apiname;
    debugPrint(method.name + " : " + url);
    debugPrint("HEADER : " + headers.toString());
    debugPrint("BODY : " + body);
    http.Response response;
    switch (method) {
      case Method.GET:
        response = await httpClient.get(Uri.parse(url), headers: headers);
        break;
      case Method.POST:
        response =
            await httpClient.post(Uri.parse(url), headers: headers, body: body);
        break;
      case Method.PUT:
        response =
            await httpClient.put(Uri.parse(url), headers: headers, body: body);
        break;
      case Method.DELETE:
        response = await httpClient.delete(Uri.parse(url),
            headers: headers, body: body);
        break;
    }
    log("RESPONSE : " + response.body);
    // SmartDialog.dismiss();
    if (response.statusCode != 200) {
      // WidgetsBinding.instance.addPostFrameCallback((_) {
      //   ScaffoldMessenger.of(context)
      //       .showSnackBar(SnackBar(content: Text("No data available")));
      // });
      // SmartDialog.showToast(response.body);

      try {
        throw Exception(json.decode(response.body)['errors'][0]['msg']);
      } catch (e) {
        throw Exception(json.decode(response.body)['error']);
      }
    }
    return jsonDecode(response.body);
  }

 @override
  Future<List<AandCDelivery>> completedDelivery() async {
    
    var headers = apiheaderWithToken;
    final body = json.encode({});
    
    return  AandCDelivery.fromJson(
            await callApi(headers, body, completedDeliveries, Method.GET))
        as List<AandCDelivery>;
    
  }

error: 'List' is not a subtype of type 'FutureOr<Map<String, dynamic>>'

API response:

[{"_id":"62e8a1351ae33236c9d75883","delivery_category_id":"62e7806795373a695265fcac","delivery_category_name":"Electronics","delivery_product_id":"62e78292320b04ecc835c755","delivery_product_name":"Mobiles","delivery_id":"62e7ea5a2e72b4ca7505a116","user_name":"Kevin","user_phone_no":"+918870457970","user_picture":"https://www.gravatar.com/avatar/866d2a555baadfb4087796588b2deac7?s=200&r=pg&d=mm","weight":"1","order_no":"EZY-41DRLE3RWL6AVNRBM"}]}]

model class:

List<AandCDelivery> aandCDeliveryFromJson(String str) =>
    List<AandCDelivery>.from(
        json.decode(str).map((x) => AandCDelivery.fromJson(x)));

String aandCDeliveryToJson(List<AandCDelivery> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class AandCDelivery {
  AandCDelivery({
    this.id,
    this.deliveryCategoryId,
    this.deliveryCategoryName,
    this.deliveryProductId,
    this.deliveryProductName,
    this.deliveryId,
    this.userName,
    this.userPhoneNo,
    this.userPicture,
    this.weight,
    this.orderNo,
  
  });

  String? id;
  String? deliveryCategoryId;
  String? deliveryCategoryName;
  String? deliveryProductId;
  String? deliveryProductName;
  String? deliveryId;
  String? userName;
  String? userPhoneNo;
  String? userPicture;
  String? weight;
  String? orderNo;
  
  factory AandCDelivery.fromJson(Map<String, dynamic> json) => AandCDelivery(
        id: json["_id"],
        deliveryCategoryId: json["delivery_category_id"],
        deliveryCategoryName: json["delivery_category_name"],
        deliveryProductId: json["delivery_product_id"],
        deliveryProductName: json["delivery_product_name"],
        deliveryId: json["delivery_id"],
        userName: json["user_name"],
        userPhoneNo: json["user_phone_no"],
        userPicture: json["user_picture"],
        weight: json["weight"],
        orderNo: json["order_no"],
      );

  Map<String, dynamic> toJson() => {
        "_id": id,
        "delivery_category_id": deliveryCategoryId,
        "delivery_category_name": deliveryCategoryName,
        "delivery_product_id": deliveryProductId,
        "delivery_product_name": deliveryProductName,
        "delivery_id": deliveryId,
        "user_name": userName,
        "user_phone_no": userPhoneNo,
        "user_picture": userPicture,
        "weight": weight,
        "order_no": orderNo,
      
      };
}

I'm getting list of values as a API response, while returning it I'm getting an error. How to clear 'List' is not a subtype of type 'FutureOr<Map<String, dynamic>>' error. ................................................................