Weather API Error: The remote server returned an error 401 unauthorized

Hello I applied the article of weather API web service https://www.aspsnippets.com/Articles/Display-daily-Weather-Forecast-using-Weather-API-in-ASPNet.aspx but it doesnt work with me he sends me back this error message  The remote server returned an error 401 unauthorized. please help me

Ligne 21 :         using (WebClient client = new WebClient())
Ligne 22 :         {
Ligne 23 :             string json = client.DownloadString(url);
Ligne 24 : 
Ligne 25 :             WeatherInfo weatherInfo = (new JavaScriptSerializer()).Deserialize<WeatherInfo>(json);