ppmnorm: 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/ppmnorm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmnorm - from package: netpbm

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

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