wpa_supplicant: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/sbin/wpa_supplicant - from package: wpa_supplicant

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

On Debian possible default full paths are:

/sbin/wpa_supplicant - from package: wpasupplicant

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