ppmtolj: 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/ppmtolj - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmtolj - from package: netpbm

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

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