Your branch is no longer directly based off of the branch you're trying to merge it into - e.g. another commit was added to the destination branch that isn't in your branch. Thus, you can't fast-forward into it (because fast-forward requires your branch to completely contain the destination branch).
You can rebase your branch on top of the destination branch (git rebase <destination branch>) to rework the commits such that they will fast forward into it, or you can do a regular merge.