Heroku build failing due to Yarn and npm lockfile conflict

I'm trying to deploy a React Web app on Heroku using the Heroku CLI. However when I run,

git push heroku master

from my project folder it throws an error as:

Counting objects: 213, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (212/212), done.
Writing objects: 100% (213/213), 515.89 KiB | 0 bytes/s, done.
Total 213 (delta 40), reused 0 (delta 0)
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://kb.heroku.com/why-is-my-node-js-build-
failing-because-of-conflicting-lock-files
remote: 
remote:  !     Push rejected, failed to compile Node.js app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !   Push rejected to MyAPP.
 remote: 
 To https://git.heroku.com/MyAPP.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 
https://git.heroku.com/MyAPP.git'

I did rm and removed the yarn lock file since I use npm. Still the same error shows up. Now when I actually do rm yarn.lock I get a no entry found in the terminal. Idk why Heroku CLI insists that I still have yarn lock file in the directory.