Si otra persona ha hecho 'push' a la misma rama que tú, Git no podrá hacer 'push' de tus cambios.
$ git push origin main
> To https://github.com/USERNAME/REPOSITORY.git
> ! [rejected] main -> main (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 details.
Para solucionarlo, obtener y fusionar los cambios realizados en la rama remota con los cambios realizados localmente:
$ git fetch origin
# Fetches updates made to an online repository
$ git merge origin YOUR_BRANCH_NAME
# Merges updates made online with your local work
O bien, puede usar git pull para ejecutar ambos comandos a la vez:
$ git pull origin YOUR_BRANCH_NAME
# Grabs online updates and merges them with your local work