ppmtoarbtxt: 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/ppmtoarbtxt - from package: netpbm-progs

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

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