darktable: 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/darktable - from package: darktable

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

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