stsadm "the server instance specified was not found"

When running an stsadm command that requires the "url" parameter, you may see the error message "the server instance specified was not found".

If you are doing this within a multi-server farm environment, consider on which server you are running the command.

Is it an application server or a web-front-end server? If it's the former, the server won't be running the "web application" service, which is required for the command to work properly.