The version I have installed is a year old (2.39 is from last year) and this is a Mac rather than windows...

    % git init
    Initialized empty Git repository in /private/tmp/gittest/.git/
    % git commit --allow-empty -m "initial"
    [main (root-commit) fa2cf4e] initial
    % git checkout -b test
    Switched to a new branch 'test'
    % git checkout -b test/1
    fatal: cannot lock ref 'refs/heads/test/1': 'refs/heads/test' exists; cannot create 'refs/heads/test/1'
    % git --version
    git version 2.39.3 (Apple Git-145)
    % git checkout -b foo/bar
    Switched to a new branch 'foo/bar'
    % git checkout -b foo/qux
    Switched to a new branch 'foo/qux'
    % git checkout -b foo
    fatal: cannot lock ref 'refs/heads/foo':     'refs/heads/foo/bar' exists; cannot create 'refs/heads/foo'

The issue is that 'test' exists before 'test/1' was (attempted to be) created.

    % pwd          
    /tmp/gittest/.git/refs/heads
    % find . -print
    .
    ./test
    ./foo
    ./foo/qux
    ./foo/bar
    ./main
    % file *
    foo:  directory
    main: ASCII text
    test: ASCII text

In refs/heads, 'test' is a file and thus git is unable to create 'test/1'.

The parent comment is specifically referring to creating a new branch from an existing branch with a slash in the name (ex: 'test-branch' and 'test-branch/new'). Trying it on my own machine (macos) yields an error. Even switching branches first and creating the new one yields an error.