pnmtoplainpnm: 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/pnmtoplainpnm - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/pnmtoplainpnm - from package: netpbm

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

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