@FlorianRD You need to be on Magic Mirror versions 2.1.0 or higher (i.e. develop branch).
Edit: What MagicMirror version are you on? 2.1.0? When you navigate from the MMM-Buttons folder to ../../node_modules you should end up in a folder with an electron folder. If there is only a electron-prebuilt folder, it should be possible to fix the issues by upgrading MagicMirror to 2.1.0.