pplot: 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/pplot - from package: xfoil

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

On Debian possible default full paths are:

/usr/bin/pplot - from package: xfoil

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

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