iwlist: 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:

/sbin/iwlist - from package: wireless-tools

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

On Debian possible default full paths are:

/sbin/iwlist - from package: wireless-tools

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

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