wpa_passphrase: 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/sbin/wpa_passphrase - from package: wpa_supplicant

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

On Debian possible default full paths are:

/usr/bin/wpa_passphrase - from package: wpasupplicant

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

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