luminance-hdr: 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/luminance-hdr - from package: luminance-hdr

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

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