Heroku deploy error - ! [remote rejected] master -> master (pre-receive hook declined) - node.js

Question

I have seen many people get this error, and I've tried every solution I've seen suggested so far. Checking if Heroku knows what kind of app it is, removing .git file from my client folder and so on. In the error message, I can see it is having trouble finding the files, but I don't know how I'd fix this as I did git add -A and git add . numerous time. I am a beginning and I am not sure what I'm missing.
This is the error message when I do git push heroku master:
Counting objects: 100% (13/13), done.
Delta compression using up to 8 threads
Compressing objects: 100% (8/8), done.
Writing objects: 100% (13/13), 13.79 KiB | 1009.00 KiB/s, done.
Total 13 (delta 0), reused 0 (delta 0), pack-reused 0
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Node.js app detected
remote:
remote: -----> Creating runtime environment
remote:
remote: NPM_CONFIG_LOGLEVEL=error
remote: NODE_ENV=production
remote: NODE_MODULES_CACHE=true
remote: NODE_VERBOSE=false
remote:
remote: -----> Installing binaries
remote: engines.node (package.json): unspecified
remote: engines.npm (package.json): unspecified (use default)
remote:
remote: Resolving node version 12.x...
remote: Downloading and installing node 12.18.3...
remote: Using default npm version: 6.14.6
remote:
remote: -----> Installing dependencies
remote: Installing node modules
remote: added 131 packages in 5.646s
remote:
remote: -----> Build
remote: Running heroku-postbuild
remote:
remote: > mern_project#1.0.0 heroku-postbuild /tmp/build_07c4cff9
remote: > NPM_CONFIG_PRODUCTION=false npm install --prefix client && npm run build --prefix client
remote:
remote: up to date in 0.242s
remote: found 0 vulnerabilities
remote:
remote: npm ERR! code ENOENT
remote: npm ERR! syscall open
remote: npm ERR! path /tmp/build_07c4cff9/client/package.json
remote: npm ERR! errno -2
remote: npm ERR! enoent ENOENT: no such file or directory, open '/tmp/build_07c4cff9/client/package.json'
remote: npm ERR! enoent This is related to npm not being able to find a file.
remote: npm ERR! enoent
remote:
remote: npm ERR! A complete log of this run can be found in:
remote: npm ERR! /tmp/npmcache.LOFwn/_logs/2020-08-28T19_01_14_938Z-debug.log
remote: npm ERR! code ELIFECYCLE
remote: npm ERR! errno 254
remote: npm ERR! mern_project#1.0.0 heroku-postbuild: `NPM_CONFIG_PRODUCTION=false npm install --prefix client && npm run
build --prefix client`
remote: npm ERR! Exit status 254
remote: npm ERR!
remote: npm ERR! Failed at the mern_project#1.0.0 heroku-postbuild script.
remote: npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
remote:
remote: npm ERR! A complete log of this run can be found in
remote: npm ERR! /tmp/npmcache.LOFwn/_logs/2020-08-28T19_01_14_958Z-debug.log
remote:
remote: -----> Build failed
remote:
remote: We're sorry this build is failing! You can troubleshoot common issues here:
remote: https://devcenter.heroku.com/articles/troubleshooting-node-deploys
remote:
remote: Some possible problems:
remote:
remote: - Node version not specified in package.json
remote: https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version
remote:
remote: Love,
remote: Heroku
remote:
remote: ! Push rejected, failed to compile Node.js app.
remote:
remote: ! Push failed
remote: Verifying deploy...
remote:
remote: ! Push rejected to floating-island-87176.
remote:
To https://git.heroku.com/floating-island-87176.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/floating-island-87176.git'

Answer 1

Add engines in package.json file
"engines": {
"node": "16.13.0",
"npm": "8.1.0"
},
Then, change version of the package.json
"version":"1.2.1"
Then
git push heroku master

Answer 2

As found in this github issue it was an error with the git repo still thinking client was a submodule. The fix is to remove the client folder from git tracking and add it back as regular files:
git rm -f --cached client && git add . && git commit -m 'Add client folder back to git repo' && git push heroku master

Answer 3

This is what worked for me (The branch is based on 'origin/master', but the upstream might be gone.
(use "git branch --unset-upstream" to fixup))
git branch --unset-upstream