If you still dont know how to sign, you can use
and select "use java key on google play" and follow instructions.
Create a key in aab signer app and add a password.
Upload the key to your computer from Phone/Downloads/File Signer/Keys
and the signed aab From Phone/Downloads/File Signer/Signed.
Once your java key is done, you can upload your app on google play. It should work.
The key should be the same as it was for me.