ppmdist: 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/ppmdist - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmdist - from package: netpbm

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

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