Fix:
First:
Make sure your "GooglePlayGamesPluginDependencies.xml" is calling the right directory.
Second:
Edit - Build Settings - Player Setting - Player - Publishing Settings -
1. Minify release : uncheck.
2. Minify debug : uncheck.
3. Costum Proguard File: enable.
4. Copy and paste the Proguard.txt (Code Below) into the proguard-user.txt file Assets\Plugins\Android\proguard-user.txt
5. Run the "Force Resolve" at Assets - External Dependency Manager - Android Resolver - Force Resolve
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proguard.txt
-keep class com.google.android.gms.games.PlayGames { *; }
-keep class com.google.android.gms.games.leaderboard.** { *; }
-keep class com.google.android.gms.games.snapshot.** { *; }
-keep class com.google.android.gms.games.achievement.** { *; }
-keep class com.google.android.gms.games.event.** { *; }
-keep class com.google.android.gms.games.stats.** { *; }
-keep class com.google.android.gms.games.video.** { *; }
-keep class com.google.android.gms.games.* { *; }
-keep class com.google.android.gms.common.api.ResultCallback { *; }
-keep class com.google.android.gms.signin.** { *; }
-keep class com.google.android.gms.dynamic.** { *; }
-keep class com.google.android.gms.dynamite.** { *; }
-keep class com.google.android.gms.tasks.** { *; }
-keep class com.google.android.gms.security.** { *; }
-keep class com.google.android.gms.base.** { *; }
-keep class com.google.android.gms.actions.** { *; }
-keep class com.google.games.bridge.** { *; }
-keep class com.google.android.gms.common.ConnectionResult { *; }
-keep class com.google.android.gms.common.GooglePlayServicesUtil { *; }
-keep class com.google.android.gms.common.api.** { *; }
-keep class com.google.android.gms.common.data.DataBufferUtils { *; }
-keep class com.google.android.gms.games.quest.** { *; }
-keep class com.google.android.gms.nearby.** { *; }