It says it is unable to obtain authorization for this operation

To build on the earlier message above...

When trying to reset the password via Apple instructions, Reset My Default Keychains dialogue box opens another dialogue box that asks for the login password, but using my current login password creates an error message "Unable to obtain authorization for this operation.

Can you provide a few details in what you’re trying to do here? Seems you're trying to change your login password on your Mac, but some more details on your computer, on the macOS software version, and on whether the computer might be bound to a directory server (or if it’s a an isolated or home computer) might help get you the answer you need.

If you’re using a Mac that is (or was recently previously) connected to a business or a school network for various purposes including password authentication and directory services, please check with the IT folks that are supporting computers connected to that etwork.