Probably you cloned your repo into Cloud9. You might need to change remote origin from https to SSH otherwise authentication errors will persist.
Use Solution # 2 from Douglas C. Ayers' Cloud9 FAQ:
git remote set-url origin YOUR_REMOTE_SSH_URL
Question
In Cloud9 whenever I try and push to a github repository with the terminal I get the following error:
fatal: Authentication failed
I am using
git push origin master
to push to the repository. I have my ssh key added in github so that is not the problem.
Solution
Probably you cloned your repo into Cloud9. You might need to change remote origin from https to SSH otherwise authentication errors will persist.
Use Solution # 2 from Douglas C. Ayers' Cloud9 FAQ:
git remote set-url origin YOUR_REMOTE_SSH_URL