How can i upload my apk on google play?

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this is the error that’s giving me i know how to upload and publish games on google play i have doing it for
a long time,but this few days it’s giving me this error why? can u help me

Read it. All of it.

In a nutshell, sign your app. Do not attempt to jump straight to that. Read the page. You likely missed a tiny step. Annoying? Yes.

Not yet valid suggests that a timestamp has not occurred yet or you have not properly registered the Key.

https://www.google.co.uk/search?q=signed+with+a+certificate+that+is+not+yet+valid&ie=utf-8&oe=utf-8&gws_rd=cr&ei=NecpVsyuMcjnUvKtjLAH