Unable to sign in to a Google account using a saved passkey

If you didn't click the "Use passkeys" option that is shown on the Google account Passkeys page, you need to enable the "Skip password when possible" option on the security settings page (it's down the bottom, after the list of device logged in).

This will then use passkeys first after entering/selecting your account, instead of password. If you need to login using a password, you can do that by clicking on the "Try another way" link shown on the on the dialog.