I was trying to https://api.dropboxapi.com/2/files/list_folder?access_token=.... but don't run. Where is wrong?

This could be a number of issues. Can you share the code snippet that calls the endpoint, and the full text of the error output? 

Error in call to API function "users/get_current_account": Must provide HTTP header "Authorization" or URL parameter "authorization".

public async Task<string> Httpclient(string link)
{
HttpClient client = new HttpClient();
HttpResponseMessage response= await client.GetAsync(link);
return await response.Content.ReadAsStringAsync();
}

public async Task<Model.Get_Current_Account.RootObject> get_current_account(string _accessToken)
{
var query = await mainViewModel.Httpclient("https://api.dropboxapi.com/2/users/get_current_account?access_token="+_accessToken);
if (query != null)
{
var get_data = JsonConvert.DeserializeObject<Model.Get_Current_Account.RootObject>(query);
return get_data;
}
else
return null;
}

You're using a URL parameter named "access_token", but per the error message that should instead be named "authorization". (Or, you can use an Authorization header.)