Hello,GDShadow,
Something related happened with me and is about Credentials.
Check the options below
1° Credentials Check:
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On Google Play Console, check if the “Google Play App Signing” is ON
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Navigate to your project directory\Build\Android and check if the certificate is the same in Google Play Consle
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Go to Google Cloud Platform? and check if the “Client ID OAuth 2.0” are the same of the Google Play Console App signing
2° Google Play Check
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Open Project settings
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Configure the settings as the Epic suggest: Using Google Play Achievements | Unreal Engine Documentation
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Go to Google Play Console in the Games Services, check if the Tests are enabled for the Alpha and the Beta.
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Upload your apk to Google Play (Alpha or Beta) and test.