In order for the debugger to know where to find your source JavaScript or TypeScript files, you'll need to specifically open up a window of Visual Studio Code relative to the behavior pack where your JavaScript or TypeScript source files are. This may be inside of Minecraft's development_behavior_packs folder (e.g., localappdata%\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\development_behavior_packs) - or you may have your source code located in a separate folder (e.g., c:\projects\myaddon).