Apresentando Erro em print(transaction)); e não salva nada.

Bom dia, estar aparecendo o seguinte erro: em print(transaction): This expression has a type of 'void' so its value can't be used.

void main() {
  runApp(BytebankApp());
  save(Transaction(200.0, Contact(0, 'Gui', 2000))).then((transaction) => print(transaction));
}


codigo webclient:

import 'dart:convert';

import 'package:bytebank/models/contact.dart';
import 'package:bytebank/models/transaction.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'package:http/http.dart';
import 'package:http_interceptor/http_interceptor.dart';

http.Client client =
    InterceptedClient.build(interceptors: [LoggingInterceptor()]);
const String baseUrl = 'http://192.168.1.152:8080/transactions';

class LoggingInterceptor implements InterceptorContract {
  @override
  Future<RequestData> interceptRequest({@required RequestData data}) async {
    debugPrint('Request');
    debugPrint('url: ${data.url}');
    debugPrint('headers: ${data.headers}');
    debugPrint('body: ${data.body}');
    return data;
  }

  @override
  Future<ResponseData> interceptResponse({@required ResponseData data}) async {
    debugPrint('Response');
    debugPrint('status code: ${data.statusCode}');
    debugPrint('headers: ${data.headers}');
    debugPrint('body: ${data.body}');
    return data;
  }
}

Future<List<Transaction>> findAll() async {
  final http.Response response =
      await client.get(Uri.parse(baseUrl)).timeout(const Duration(seconds: 5));
  final List<dynamic> decodedJson = jsonDecode(response.body);
  final List<Transaction> transactions = List();
  for (Map<String, dynamic> transactionJson in decodedJson) {
    final Map<String, dynamic> contactJson = transactionJson['contact'];
    final Transaction transaction = Transaction(
      transactionJson['value'],
      Contact(
        0,
        contactJson['name'],
        contactJson['accountNumber'],
      ),
    );
    transactions.add(transaction);
  }
  return transactions;
}

Future<void> save(Transaction transaction) async {
  final Map<String, dynamic> transactionMap = {
    'value': transaction.value,
    'contact': {
      'name': transaction.contact.name,
      'accountNumber': transaction.contact.accountNumber
    }
  };
  final String transactionJson = jsonEncode(transactionMap);
  client.post(Uri(host: baseUrl),
      headers: {
        'Content-type': 'application/json',
        'password': '1000',
      },
      body: transactionJson);
  final Response response = await client.post(Uri(host: baseUrl),
      headers: {
    'Content-type' : 'application/json',
    'password' : '1000'
  },
      body: transactionJson);
  Map<String, dynamic> json = jsonDecode(response.body);
  final Map<String, dynamic> contactJson = json['contact'];
  return Transaction(
    json['value'],
    Contact(
      0,
      contactJson['name'],
      contactJson['accountNumber'],
    ),
  );
}