Heroku "failed to push some refs" when trying to push React app

Going by this: https://dev.to/smithmanny/deploy-your-react-app-to-heroku-2b6f, I am trying to deploy a simple react app out of the box from create-react-app to Heroku.

npx create-react-app my-app
cd my-app
npm install

Then add these to package.json:

"engines": {
    "npm": "6.12.0",
    "node": "10.16.3"
},

Then:

heroku login blah...
git init
git add .
git commit -m "initial commit"
heroku create
git remote add heroku https://git.heroku.com/damp-spire-48480.git (auto-generated name of app)
git push heroku master

Then I get this:

Enumerating objects: 27666, done.
Counting objects: 100% (27666/27666), done.
Delta compression using up to 8 threads
Compressing objects: 100% (20069/20069), done.
Writing objects: 100% (27666/27666), 24.31 MiB | 362.00 KiB/s, done.
Total 27666 (delta 5987), reused 27666 (delta 5987)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Node.js app detected
remote: 
remote: -----> Build failed
remote:  !     Two different lockfiles found: package-lock.json and yarn.lock
remote: 
remote:        Both npm and yarn have created lockfiles for this application,
remote:        but only one can be used to install dependencies. Installing
remote:        dependencies using the wrong package manager can result in missing
remote:        packages or subtle bugs in production.
remote: 
remote:        - To use npm to install your application's dependencies please delete
remote:          the yarn.lock file.
remote: 
remote:          $ git rm yarn.lock
remote: 
remote:        - To use yarn to install your application's dependences please delete
remote:          the package-lock.json file.
remote: 
remote:          $ git rm package-lock.json
remote:     
remote:        https://help.heroku.com/0KU2EM53
remote: 
remote:  !     Push rejected, failed to compile Node.js app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !   Push rejected to damp-spire-48480.
remote: 
To https://git.heroku.com/damp-spire-48480.git
 ! [remote rejected]   master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/damp-spire-48480.git'

I've looked at a few SO pages now, and so far not seeing a solution. I should not that I've had a few apps and remotes up there, but I think I've removed all app and remotes, and tried to start from scratch... Unless there's something else to clean up...

I tried this one: Push to Heroku denied - "failed to push some refs to 'heroku". So I tried: git push heroku master:master and git push heroku HEAD:master and got the same result each time.

Solution

If there was one in there, and you remove it later, then it still fails.

It should not, if you remove it and commit, recording the deletion:

As explained in Heroku page "Why is my Node.js build failing because of conflicting lock files?"

git rm yarn.lock
git commit -m "Remove yarn lock file"
git push heroku master

And/or

git rm package-lock.json
git commit -m "Remove npm lock file"
git push heroku master