"run: not found" Error When Deploying MERN App to Heroku - node.js

Question

I am trying to deploy my MERN web application to Heroku, but I have been getting this error:
remote: sh: 1: run: not found
remote: npm ERR! code ELIFECYCLE
remote: npm ERR! syscall spawn
remote: npm ERR! file sh
remote: npm ERR! errno ENOENT
remote: npm ERR! reach-app#1.0.0 heroku-postbuild: `NPM_CONFIG_PRODUCTION=false npm install npm && run build`
remote: npm ERR! spawn ENOENT
remote: npm ERR!
remote: npm ERR! Failed at the reach-app#1.0.0 heroku-postbuild script.
remote: npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
In terms of my structure, I have two directories in my root directory, called client and backend. Both of these directories each have their own package.json. I also initlialised git and another npm in the root directory. So, could it possibly be a problem with my file structure?
It doesn't seem like there are any other similar questions out there, so any help would be greatly appreciated. Thanks
EDIT:
This is my package.json in my root directory:
{
"name": "reach-app",
"version": "1.0.0",
"description": "Initialised npm for the root directory.",
"main": "index.js",
"scripts": {
"start": "node backend/server/index.js",
"heroku-postbuild": "NPM_CONFIG_PRODUCTION=false npm install && npm run build",
"test": "echo \"Error: no test specified\" && exit 1"
},
"keywords": [
"example",
"heroku"
],
"author": "Phillip La",
"license": "ISC",
"engines": {
"node": "12.16.2"
}
}