ppmtouil: 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 CentOS possible default full paths are:

/usr/bin/ppmtouil - from package: netpbm-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ppmtouil - from package: netpbm

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

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