Slackpkg?
 

root@frank:/home/frank# slackpkg update firefox

You need to upgrade your slackpkg.conf.
This is a new slackpkg version and many changes happens in config files.
In /etc/slackpkg/slackpkg.conf.new there is a sample of the new configuration.

After update your configuration run: slackpkg update

How do I upgrade slackpkg.conf.?

i don't use slackpkg, but i am willing to guess the files are in /etc

look for two files slackpkg.conf and slackpkg.conf.new or something like that, save the origional one (slackpkg.conf) somewhere safe like in /root or /home and rename slackpkg.conf.new to slackpkg.conf...

or you can rename slackpkg.conf to slackpkg.conf-OLD

read the man page will you?

slackpkg update is for upgrading the filelists
usage is pretty self-explanatory (slackpkg install or slackpkg remove or etc.)

That's pretty much telling you that the /etc/slackpkg/slackpkg.conf file present on your system will not work with the new version of slackpkg that you installed, so you need to remove that /etc/slackpkg/slackpkg.conf and copy over the /etc/slackpkg/slackpkg.conf.new in its place. After doing that, you might want to look at the file to make sure there's not any customization needed.