xfoil: 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/xfoil - from package: xfoil

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

On Debian possible default full paths are:

/usr/bin/xfoil - from package: xfoil

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

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