If you are using lpadmin I gather you have cups installed. I ran into this and -- after a considerable investment in time -- decided it must be a problem within the command. Once -- maybe twice -- I did not get that error message but the printer didn't work. The only reliable way I've found, to add a printer, is to use the browser interface.
But there is an alternative, which is to determine which cgi script, from the cups package(s) are used to perform a given task. Use the View-Source menu entries to get started.
With a browser running as non-root (because browsing as root, even locally, is a bad habit to start), browse to http://127.0.0.1:631/.
When you attempt to do something from within this menu, that is priviledged, you will be prompted for a username / password. If you haven't defined users with CUPS admin privs yet, you'll need to enter "root" as the username and the appropriate password.
This will allow you to create a CUPS printer, view existing printers that Cups knows about, configure them, etc.
If you are using HP hardware -- like jetdirect or one of their printers, or if you to try the following for fun: try and find the slackware equivalent of the HPOJ package. The command "ptal-device" is a comand-line tool for returning available CUPS printer names.
Hope this helps.