slack: command not found

Have You tried accessing the binary with the full path?

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/sbin/slack - from package: slack

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