Why unity 5.5 not support "using GooglePlayGames?

help me please.......
I try this script in unity 5.5 but that doesnt support.
mynotebook just suppoort unity 5.5, but i need add a leaderboard to my game.
what should I do?

Code (csharp):

1.  
2. using GooglePlayGames;
3. using GooglePlayGames.BasicApi;
4. using UnityEngine;
5.  
6. public class PlayGamesScript : MonoBehaviour {
7.  
8. // Use this for initialization
9. void Start () {
10.         PlayGamesClientConfiguration config = new PlayGamesClientConfiguration.Builder().Build();
11.         PlayGamesPlatform.InitializeInstance(config);
12.         PlayGamesPlatform.Activate();
13.  
14.         SignIn();
15. }
16.  
17.     void SignIn()
18.     {
19.         Social.localUser.Authenticate(success => { });
20.     }
21.  
22.     #region Achievements
23.     public static void UnlockAchievement(string id)
24.     {
25.         Social.ReportProgress(id, 100, success => { });
26.     }
27.  
28.     public static void IncrementAchievement(string id, int stepsToIncrement)
29.     {
30.         PlayGamesPlatform.Instance.IncrementAchievement(id, stepsToIncrement, success => { });
31.     }
32.  
33.     public static void ShowAchievementsUI()
34.     {
35.         Social.ShowAchievementsUI();
36.     }
37.     #endregion /Achievements
38.  
39.     #region Leaderboards
40.     public static void AddScoreToLeaderboard(string leaderboardId, long score)
41.     {
42.         Social.ReportScore(score, leaderboardId, success => { });
43.     }
44.  
45.     public static void ShowLeaderboardsUI()
46.     {
47.         Social.ShowLeaderboardUI();
48.     }
49.     #endregion /Leaderboards
50.  
51. }
52.  

 

I believe for this to work you have to have this in your project https://github.com/playgameservices/play-games-plugin-for-unity

 

so you mean i must import that package before i am use " using GooglePlayGames;" in my game?

 

when im add google play services package. this problem arise. would you explant what's wrong?