A non-fast-forward error happens when the remote repository has commits you don’t have locally. Fetch and merge or rebase to sync with the remote.

The 'non-fast-forward' error on GitHub occurs because your branch is out of sync with the remote branch, meaning the remote branch has commits that your local branch doesn’t. This often happens when another collaborator has pushed changes to the branch you’re working on. To resolve this, start by using git fetch to bring the latest remote changes into your local repository. Then, you have two main options: merging or rebasing. To merge, use git merge origin/branch-name to combine the changes from the remote branch with yours, creating a new commit that reconciles both versions. If you prefer rebase, use git rebase origin/branch-name to apply your commits on top of the fetched commits, which keeps your history linear. Once your local branch is updated, you should be able to push without encountering a fast-forward error. Remember, if you are unsure about which approach to use, merging is generally safer as rebasing rewrites history, which can affect collaboration.