const fs = require('fs');
const folderName = process.argv[2] || 'Project'
fs.mkdirSync(folderName)
fs.writeFileSync(`${folderName}/index.html`)
fs.writeFileSync(`${folderName}/style.css`)
fs.writeFileSync(`${folderName}/app.js`)
This is the error I am getting when I execute:
node boilerplate.js Portfolio
the folder "Portfolio" is made but not the files😓

Have you looked at the documentation for fs.writeFileSync?
It says fs.writeFileSync(file, data[, options]) – the brackets denote optional parameters and that means only options is optional, so you must always pass in file and data.
You aren't passing data, and the error message alludes to that with undefined.
If you want empty files, pass in an empty string as data.
fs.writeFileSync(`${folderName}/index.html`, '')
fs.writeFileSync(`${folderName}/style.css`, '')
fs.writeFileSync(`${folderName}/app.js`, '')