I was able to get mix working just fine on my Laravel 5.3 build with Windows.
I also had a an issue with my manifest file not being found on my first attempt.
So the issue I was having was in my configuration. I read through the docs carefully because I was having issues and made sure to update my package.json properly.
Also I attempted to do npm prune and then run npm install but this didn't work exactly right for me. So my solution was to manually delete the node_modules folder and then run npm install
Finally you said that your error was 'Unable to locate Mix file:/css/app.css. ' is your webpack.mix.js file configured correctly? Make sure you update the default 'src/css/app.css' to your 'resources/css/app.css' or whatever your trying to mix.
Lastly there is a note in the docs that you need to call mix.js() at least once in your mix file. Are you trying to compile just css without calling .js() at least once?