box market place app authentication

hi

I am having trouble understanding authentication for my box application that I intend to add in box marketplace.

I understand how to authenticate in Box using OAuth 2.0 with JWT , but if continue with this approach the user himself will need to create a developer app.

I understand this might be basic question but ,

I want to understand how through my app and one time user authentication , I can access user content  without requiring user intervention again?

If I go with OAuth 2.0 with JWT , I face this error while trying to authenticate "The grant type is unauthorized for this client_id".

If I go with Standard OAuth 2.0 , I face this error when trying to get folder items from a different user other than admin "The request requires higher privileges than provided by the access token". 

Can anyone help me in this regard ?
attaching the test code i am using .

    public class BoxController : Controller
    {
        
        private const string BoxClientId = "xxxxxxxxxxxxxxxxxxxxx";
        private const string BoxSecret = "xxxxxxxxxxxxxxxxxxxxx";
        private const string RedirectUri = "http://localhost:56742/Box/Response/";


        private static Tuple BuildConfig()
        {
            var config = new BoxConfig(BoxClientId,BoxSecret,new Uri(RedirectUri));
            var client = new BoxClient(config);

            return Tuple.Create(config, client);
        }

        public RedirectResult Index()
        {
            var config = BuildConfig().Item1;

            return Redirect(config.AuthCodeUri.ToString());
        }

        public async Task Response()
        {
            var client = BuildConfig().Item2;
            var code = Request.QueryString["code"];
            var authsesion = await client.Auth.AuthenticateAsync(code);


            var accesstoken = authsesion.AccessToken;
            var refreshtoken = authsesion.RefreshToken;
            var expiresin = authsesion.ExpiresIn;

            using (var stream = new FileStream("tax.docx", FileMode.Open))
            {
                var req = new BoxFileRequest()
                {
                    Name = "boxtax.docx",
                    Parent = new BoxRequestEntity() { Id = "0" }
                };

                var newFile = await client.FilesManager.UploadAsync(req, stream);
                Console.Out.WriteLine(newFile.Id);
            }

            
            var boxusercollection = await client.UsersManager.GetEnterpriseUsersAsync(null, 0, 500);
            var userlist = boxusercollection.Entries.ToDictionary(item => item.Id, item => item.Name + ";" + item.Login);


            // get a boxclient using accesstoken and refresh token for a different user

            var auth2 = new OAuthSession(accesstoken, refreshtoken, 3600, "bearer");

            var config = BuildConfig().Item1;

            var key = userlist.First().Key;
            var userClient = new BoxClient(config, auth2, asUser: key);
            var items = await userClient.FoldersManager.GetFolderItemsAsync("0", 500);

            var itemList = items.Entries.ToDictionary(item => item.Id, item => item.Name);

            Console.Out.WriteLine(itemList.ToString());

        }
    }