Your configuration specifies to merge with the <branch name> from the remote, but no such ref was fetched.?

I am getting this error for pull:

Your configuration specifies to merge with the ref 'refs/heads/feature/Sprint4/ABC-123-Branch' from the remote, but no such ref was fetched.

This error is not coming for any other branch.
The special thing about this branch is that it is created from the previous commit of another branch.

My config file looks like:

[core]
    repositoryformatversion = 0
    filemode = false
    bare = false
    logallrefupdates = true
    symlinks = false
    ignorecase = true
    hideDotFiles = dotGitOnly
[remote "origin"]
    url = <url here>
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    remote = origin
    merge = refs/heads/master
[branch "new-develop"]
    remote = origin
    merge = refs/heads/new-develop
[branch "feature/Sprint4/ABC-123-Branch"]
    remote = origin
    merge = refs/heads/feature/Sprint4/ABC-123-Branch

Solution

What this means

Your upstream—the remote you call origin—no longer has, or maybe never had (it's impossible to tell from this information alone) a branch named feature/Sprint4/ABC-123-Branch. There's one particularly common reason for that: someone (probably not you, or you'd remember) deleted the branch in that other Git repository.

What to do

This depends on what you want. See the discussion section below. You can:

create or re-create the branch on the remote, or
delete your local branch, or
anything else you can think of.

Discussion

You must be running git pull (if you were running git merge you would get a different error message or no error message at all).

When you run git fetch, your Git contacts another Git, based on the url line under the [remote "origin"] section of your configuration. That Git runs a command (upload-pack) that, among other things, sends your Git a list of all branches. You can use git ls-remote to see how this works (try it, it is educational). Here is a snippet of what I get when running this on a Git repository for git itself:

$ git ls-remote origin
From [url]
bbc61680168542cf6fd3ae637bde395c73b76f0f    HEAD
60115f54bda3a127ed3cc8ffc6ab6c771cbceb1b    refs/heads/maint
bbc61680168542cf6fd3ae637bde395c73b76f0f    refs/heads/master
5ace31314f460db9aef2f1e2e1bd58016b1541f1    refs/heads/next
9e085c5399f8c1883cc8cdf175b107a4959d8fa6    refs/heads/pu
dd9985bd6dca5602cb461c4b4987466fa2f31638    refs/heads/todo
[snip]

The refs/heads/ entries list all of the branches that exist on the remote,¹ along with the corresponding commit IDs (for refs/tags/ entries the IDs may point to tag objects rather than commits).

Your Git takes each of these branch names and changes it according to the fetch line(s) in that same remote section. In this case, your Git replaces refs/heads/master with refs/remotes/origin/master, for instance. Your Git does this with every branch name that comes across.

It also records the original names in the special file FETCH_HEAD (you can see this file if you peek into your own .git directory). This file saves the fetched names and IDs.

The git pull command is meant as a convenience short-cut: it runs git fetch on the appropriate remote, and then git merge (or, if so instructed, git rebase) with whatever arguments are needed to merge (or rebase) as directed by the [branch ...] section. In this case, your [branch "feature/Sprint4/ABC-123-Branch"] section says to fetch from origin, then merge with whatever ID was found under the name refs/heads/feature/Sprint4/ABC-123-Branch.

Since nothing was found under that name, git pull complains and stops.

If you run this as two separate steps, git fetch and then git merge (or git rebase), your Git would look at your cached remotes/origin/ remote-tracking branches to see what to merge with or rebase onto. If there was such a branch at one time, you may still have the remote-tracking branch. In this case, you would not get an error message. If there was never such a branch, or if you have run git fetch with --prune (which removes dead remote-tracking branches), so that you have no corresponding remote-tracking branch, you would get a complaint, but it would refer to origin/feature/Sprint4/ABC-123-Branch instead.

In either case, we can conclude that feature/Sprint4/ABC-123-Branch does not exist now on the remote named origin.

It probably did exist at one time, and you probably created your local branch from the remote-tracking branch. If so, you probably still have the remote-tracking branch. You might investigate to see who removed the branch from the remote, and why, or you might just push something to re-create it, or delete your remote-tracking branch and/or your local branch.

¹Well, all that it is going to admit to, at least. But unless they have specifically hidden some refs, the list includes everything.

Edit, Jul 2020: There's a new fetch protocol that can avoid listing everything, and only list names that your Git says it's looking for. This can help with repositories that have huge numbers of branches and/or tags. However, if your Git is interested in all possible names, you'll still get all the names here.