pnmtorast: 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/pnmtorast - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmtorast - from package: netpbm

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

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