The error is pretty straightforward. Most likely the reason is that your index.html file is not in the root directory.
Or if it is in the root directory then the relative referencing is not working.
So you need to tell the server exact location of your file. This could be done by using dirname method in NodeJs. Just replace your code with this one:
app.get('/', function(req, res){
res.sendFile(__dirname + '/index.html');
});
Make sure that your add the slash "/" symbol before your homepage. Otherwise your path will become: rootDirectoryindex.html
Whereas you want it to be: rootDirectory/index.html