ppmtoexr: 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/ppmtoexr - from package: exrtools

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

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