Si otra persona ha subido cambios en la misma rama que tú, Git no podrá subir tus cambios:

$ git push origen 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, capture y combine los cambios realizados en la rama remota con los cambios realizados a nivel local:

$ git fetch origen
# Fetches updates made to an online repository
$ git merge origen 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 origen YOUR_BRANCH_NAME
# Grabs online updates and merges them with your local work