TypeError: The 'compilation' argument must be an instance of Compilation

What do your files look like?

Mix v6 package.json scripts should look like this:

"scripts": {
    "development": "mix",
    "watch": "mix watch",
    "watch-poll": "mix watch -- --watch-options-poll=1000",
    "hot": "mix watch --hot",
    "production": "mix --production"
}

And seeing that you are using Vue, in Mix v6 you need to tell mix that you will use Vue

mix.js('src/app.js', 'dist').vue();