darktable-viewer: 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-viewer - from package: darktable

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

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