No. IconChamp does not run on macOS 10.15 and below.
If you want to try theming your icons on those versions check out LiteIcon
Try reopening IconChamp and applying the changes again. Also send us bug report with the crash logs if you can.
You might also try opening the preferences and clicking `Reset Icon Caches`.
Currently you'll have to find them or make them on your own. A great place to get some free macOS icons is at macosicons.com
We won't know for sure until we get our hands on it but for now we hope to be able to keep updating IconChamp when new versions of macOS are released.