ppmtopj: 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/ppmtopj - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtopj - from package: netpbm

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

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