Try updating your ca-certificates package. The ca-certificates-20211005-noarch-1_slack14 package contains the ISRG Root X1 certificate, which seems to be missing from your system.
What's the current version of ca-certificates on your system? ca-certificates-20130906-noarch-1 by any chance?
Because when I search for ca-certificates for Slackware 14.1, I get two results: The Slackware Linux Project: Packages An ancient version and a recent version. But the "location" of the recent version is "patches/packages", whatever that may be. Maybe you need to enable "patches" first? (I don't have the slightest clue about Slackware... Didn't even know it still existed..)