[git] remote rejected master -> master (pre-receive hook declined)

I also faced the same issue The solution for this is

Delete the package-lock.json file on both the client and server

Make sure that you are using at least the LTS version of Node locally

Make sure your local Node version matches what is shown in the package.json file's engines property.

Run the git add . , git commit -m "fixing versions" and git push heroku master commands to force a rebuild.