ppmtopjxl: 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/ppmtopjxl - from package: netpbm-progs

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

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