There really haven't been any serious rootkit attacks against macOS in years, mostly due to System Integrity Protection provided in recent OS versions that prevent such things. Note that chkrootkit contains almost no macOS checks, only a few of the Darwin unix commands.
If you downloaded the source directly from and compiled it, then yes, it does require that Apple CommandLineTools is installed. The current version of chkrootkit is 0.53 dated Feb 11 2019. Where did you obtain checkrootkit from and what version? Are you using something like MacPorts or Homebrew or some other site. I still get that error when checking today.
The latest version is 15.0.4 dated Feb 9 of this year, so it should not need to be updated. As to the error you see when checking for an update, I reported that to them when I had 15.0.0 and never heard back, so have been re-downloading since then. I'll guess that you downloaded Catalina Cache Cleaner directly from Northernsoftware back in March.
