For those who have similiar problem, follow these steps :
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Go to Google Developer Console
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Click All Applications > Your Application
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Go to Release Management > App Signing
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Notice there is an " Upload Certificate" and an " App Signing Certificate".
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Copy the SHA1 from the APP SIGNING CERTIFICATE.
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Click All Applications on the Top Left, then click Game Services > Game Details
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Scroll all the way down and look for
API Console project
This game is linked to the API console project called ‘YOUR APPLICATION NAME SHOULD BE HERE’ -
Click on your application name at this location.
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Now you’re in the API’s & services. Click on Credentials
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Edit your OAuth 2.0 client ID.
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COPY THE SHA1 FROM THE APP SIGNING CERTIFICATE HERE AND BE SURE YOUR PACKAGE NAME IS CORRECT.