Description:
This error is faced when git cannot commit your changes to the remote repository. This may happen because your commit was lost or if someone else is trying to push to the same branch as you. This is the error you face.

$ git push origin master

To https://github.com/USERNAME/REPOSITORY.git

! [rejected]   master -> master (non-fast-forward)

error: failed to push some refs to 'https://github.com/USERNAME/REPOSITORY.git'

To prevent you from losing history, non-fast-forward updates were rejected

Merge the remote changes (e.g. 'git pull') before pushing again.  See the

'Note about fast-forwards' section of 'git push --help' for detail

Solution: git pull --rebase origin master