git-commits-since: 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-commits-since - from package: git-extras

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

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