ppmtopcx: 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/ppmtopcx - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtopcx - from package: netpbm

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

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