I keep trying to render my 1.12.2 world with no mods, but BlueMap keeps telling me the render threads are idle! How rude! Even when I force-render the world, it keeps those dang diddly render threads cold! Needless to say, BlueMap is being quite uncouth! I've prepared a moving picture demonstration here: https://drive.google.com/file/d/168THFOQRBoST45kPSswU5TT856SUHW6b/view?usp=sharing

I suggest that you join the Discord server if possible for support on using BlueMap, it's much easier to communicate that way than GitHub issues.
Here's the link: https://bluecolo.red/map-discord

I figured it out. I'm running 1.12.2. I'm crying currently.

1.12.2 isn't supported, not sure what version supported that, iirc Bluemap 1.5.5 or lower, idk

Latest release that can be run on 1.12.2 servers https://github.com/BlueMap-Minecraft/BlueMap/releases/tag/v1.5.5