Cannot 'git push' to Heroku

Looks like it might be an issue with your security certificate. Heroku doesn't support ECDSA key, per their documentation. I would create a fresh RSA SSH key that you use for heroku and only for heroku, send that new public key to them, then setup a ssh config file to serve that key when SSH reaches out to heroku.com.

This is what I have in my ~/.ssh/config file on my laptop:

Host heroku.com
  IdentityFile /Users/danielrice/.ssh/identity.heroku.danielricecodes
  IdentitiesOnly yes