Hello Amel,
You don't need to run NPM to recompile LESS files. All you need to do is download the starter child theme, install it and active development mode under Admin->Appearance->Settings. You are then ready to start writting in LESS in your child theme style.less file. Beans will automatically recognize file changes, therefore it will automatically recompile when you make changes (in dev mode only) and process it for you 🙂
You will find inline comments in your child theme which should be pretty self-explanatory. I will glady answer any other question you may have 🙂
Happy coding,