DioError Http status error [401] while displaying data - flutter

Question

I'm trying to display data from the API to the screen, but I can't do it because of the 401 error, take a look at my code and tell me what exactly is wrong, I think that I wrote the API incorrectly. At the moment I am trying to find the information myself, but I think the problem is in the API, and if so, what exactly is the problem?
Code :
API:
class ApiService {
Dio dio = new Dio();
var token ="token";
var refresh_token ="token";
Future getUserCards() async {
try {
Response resp;
var get_cards = "https://example/api/cards";
resp = await dio.get(get_cards);
dio.options.headers["Authorization"] = "Bearer ${token}";
dio.options.headers['Content-Type'] = "application/json";
var json = (resp.data);
var value = json["id"]["row"]["seq_num"]["text"];
return value;
} catch (e) {
print(e);
}
Future loginUser(String username, String password) async {
var storage = new FlutterSecureStorage();
await storage.write(key: 'JWT', value: token);
var login = "https://example/users/login/";
final data = {"username": username, "password": password};
Response response;
response = await dio.post(login, data: data);
dio.options.headers["Authorization"] = "Bearer ${token}";
dio.options.headers['Content-Type'] = "application/json";
if (response.statusCode == 200) {
Get.to(CardScreen());
return response.data;
} else if (response.statusCode == 401) {
var refreshToken = await dio.post(
"https://example.api/cards/refresh/");
response = await dio.post(refresh_token, data: data);
dio.options.headers["Authorization"] =
"Bearer ${token},'Content-Type': 'application/json','refresh_token': '$refresh_token'";
storage = response.data["token"];
refresh_token = response.data["refresh_token"];
return loginUser("username", "password");
} else
return null;
}
}
UI :
children: [
Expanded(
child: FutureBuilder<dynamic>(
future: ApiService().getUserCards(),
builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
if (!snapshot.hasData) {
return Center(
child: CircularProgressIndicator(),
);
} else {
return ListView.builder(
itemCount: snapshot.data.length,
itemBuilder: (context, index) {
return Card(
child: Text(snapshot.data[index]),
);
});
}
},
)