If you're letting Google manage the signing of the app, you only need an upload key and then you need to sign the bundle using the same upload key and upload the bundle

Go to Android studio's 'Build' - 'Generate Signed Bundle / Apk' - Choose ' Android App Bundle' - If you already have a key choose it or create new and sign the bundle then upload it

Upload and release keys are different. Once you choose google play signing you always sign your bundle with upload key and you don't have to worry about release key anymore. Play will check the signature of the uploaded bundle to make sure it's signed using the same upload key and if it is, they will use release key they've with them