Do not use Safari on Sierra 10.12.6 for banking or sensitive logins. Apple stopped signing security patches in 2019. Instead, install Firefox 115 ESR (Extended Support Release) or Chromium Legacy , both of which still receive updates for Sierra.
To download macOS Sierra 10.12 from the Internet Archive: mac os sierra 1012dmg download free
softwareupdate --fetch-full-installer --full-installer-version 10.12.6 The installer will download directly into your Applications Apple Support Critical Tips Compatibility Check Do not use Safari on Sierra 10