How to upload a signed .aab file in play store using visual studio

This is my first app to publish and I have tried for over a week now with no success. I have tried two ways to publish an app on play store.

1. Through "register Google API access" but I received an error message error 400: invalid_request indicating that the oAuth 2.0 rule for security is not respected
2. I also tried to upload an .aab bundle from Google console in "testing". I received another error message that the bundle is not signed

I have tried many other post with no success. Can someone help? Am really desperate.