"No java class com.amazon.ags.api.unity.AmazonGamesClientProxyImpl present"

Hi, I'm a Unity developer and it's my first experience with Amazon SDK. I imported the AmazonGameCircle.unitypackage, changed the manifest and api_key files and (apparently) initialized the service with sucess because I received a "AmazonGameCircle: Client GameCircle - Service is ready" message on my console. But I also received a "AmazonGameCircle warning: No java class com.amazon.ags.api.unity.AmazonGamesClientProxyImpl present, can't use AGSClient" message and (obviously) things don't work. What I missed? Thanks in advance, Kazo.

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