slack-diff: 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/slack-diff - from package: slack

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

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