git-changelog: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/git-changelog - from package: git-extras

You may install the required package with command apt-get install {package}

Error in other languages:
git-changelog: bevel nie gevind nie
git-changelog: comandă negăsită
git-changelog: comando não encontrado
git-changelog: commande introuvable
git-changelog: command not found
git-changelog: käsku ei ole
git-changelog: không tìm thấy lệnh
git-changelog: komanda nerasta
git-changelog: Kommando nicht gefunden.
git-changelog: kommandot finns inte
git-changelog: komut yok
git-changelog: nie znaleziono polecenia
git-changelog: níor aimsíodh an t-ordú
git-changelog: no se encontró la orden
git-changelog: no s'ha trobat l'ordre
git-changelog: opdracht niet gevonden
git-changelog: parancs nem található
git-changelog: perintah tidak ditemukan
git-changelog: príkaz nenájdený
git-changelog: příkaz nenalezen
git-changelog: команда не найдена
git-changelog: командата не е открита
git-changelog: 命令找不到
git-changelog: コマンドが見つかりません