confusing “Upload failed” message in Google Play Console

Question

I would like to add a new App to Google Play. I have already some apps signed and published to google play. All with the same key. Now with the new one I get the message :

Answer 1

In Google Play App signing Google Play signs the app for you. You should upload signed with a new key, called the upload key.

If you really want it signed with the same key as your other apps (I wouldn't recommend this) then you should opt-out of Google Play app signing for this app.