Weird. Can you check the output of “Command Palette (CMD-Shift-P)” → "DEBUG ReScript: Dump LSP Server State "
Especially the bscBinaryLocation, e.g.:
"bscBinaryLocation": "/Users/florian/stuff/test-rescript-12/node_modules/@rescript/darwin-arm64/bin/bsc.exe",
If that is somehow not set try to set the platform path in the workspace settings:
"rescript.settings.platformPath": "node_modules/@rescript/darwin-arm64/bin",
Adjusted for the OS and platform you use:
@rescript/darwin-arm64@rescript/darwin-x64@rescript/linux-arm64@rescript/linux-x64@rescript/win32-x64