pxplot: 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/pxplot - from package: xfoil

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

On Debian possible default full paths are:

/usr/bin/pxplot - from package: xfoil

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

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