I tried this command on both our 10.5.1 test environment and our 10.3.3 prod environment and received the same result that you did.
However, it seems that you can reset the password using this. Maybe this helps ? Resetting the Administrator Password or Account | Tableau Software
Hi Jeffrey, the documentation says that service.runas.password is stored temporarily. My guess is that it's needed only during installation (or when changing the configuration) and is subsequently removed.