ppmntsc: 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/ppmntsc - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmntsc - from package: netpbm

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

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