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

This error message is indicating that the push operation you are trying to perform is being rejected because it is not a fast-forward merge.

A fast-forward merge can occur when the branch you are trying to push is ahead of the remote branch you are pushing to. In this case, Git can simply move the remote branch pointer to the new commit.

However, if the remote branch has new commits that are not present in your local branch, you cannot perform a fast-forward merge. This is because doing so would overwrite the new commits on the remote branch.

To resolve this error, you will need to pull the changes from the remote branch into your local branch first, resolve any conflicts, and then push your changes again. Alternatively, you can force push your changes, but this should only be done if you are certain that it will not overwrite any changes on the remote branch.