When I try to use select brush, this appears :
Unknown function
worldtool:ui/tool_brush/select_brush/structure
Instead of structure you can put any kind of brush.
Using 21w05b

WorldTool doesn't currently work in snapshots as-is. Have you followed the steps I've listed to make it work in 1.17?

If you have any further questions, feel free to ask on my discord. The link's in the readme.
I'll close this now, as I'd prefer keeping issues as bugs and perhaps feature requests.