React / Express - Error: ENOENT: no such file or directory, stat '/app/build/index.html' - node.js

Question

Please see edits below
Please put me out of my misery here. I've spent hours looking through docs and trying different approaches found on this site. I am getting this error when heroku tries to build after pulling code from github:
Error: ENOENT: no such file or directory, stat '/app/build/index.html'
I have my client code in root and my express server code in /server.
Node is being started from the root package.json (e.g. node server/index.js).
If I bash into heroku I can see the /build/index.html file.
app.use(express.static(path.resolve(__dirname, '../build')));
app.get('*', (req, res) => {
res.sendFile(path.resolve('build/index.html'));
});
EDIT
Something else that is curious. If I set the path like so I can browse my images in /build folder on localhost:5000 (same path off of root) but I still get the same error in prod.
app.use(express.static(path.join(__dirname, '/../build')));
EDIT 2:
It turns out that I needed to add a static reference to 'public'. Argh!
I can now see index and anything else in the build directory working.
However, there is no reference to the static/js files that are created during the build and thus the page is empty. I can see them on the server in bash prompt.
app.use(express.static(path.join(__dirname, '..', 'build')));
app.use(express.static('public'));
app.use((req, res, next) => {
res.sendFile(path.join(__dirname, '..', 'build', 'index.html'));
});

Answer 1

I never did find the issue so I scaled it back and had another go of it. Ultimately I don't think you even need to reference the index.html file. At the root level I have server.js. In that file I simply reference the client/build after my route files and this acts as the default for anything not found in the route:
app.use(express.static("client/build"));

Answer 2

I hope this will help other people with this issue. I ran into this error this week and I solved it. In my case it has nothing to do with the client/build/index.html path. And I bet it's the same thing in your case as well.
Please bear with me and go through these points first to understand where the problem comes from:
The problem is that when you create your react app, it somehow runs a git init command automatically. You end up with 2 git folders: one for the Express backend and another for your react client folder.
The result is that when you do commit your changes, those in the client folder are not being pushed to heroku.
To convince yourself that this is the case, run these commands in your terminal at the root of your project:
a) heroku run bash
b) cd into the client folder
c) ls client
if at this stage you don't see the content of your client folder, then that's what caused the problem in the first place.
Now for the remedy:
Go to your terminal and run these commands (credit goes to: Content of create-react-app is not pushed in github)
mv client subfolder_tmp
git submodule deinit client
git rm --cached client
mv subfolder_tmp client
git add client
now make changes in your client folder and then push them to heroku (git add . , git commit -m "", git push heroku master)
Let me know if this helps, I know I solved my problem this way.