hpcdtoppm: 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/hpcdtoppm - from package: netpbm-progs

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

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