ppmtoterm: 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/ppmtoterm - from package: netpbm-progs

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

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