asterisk: 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/asterisk - from package: asterisk

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

On Debian possible default full paths are:

/usr/sbin/asterisk - from package: asterisk
/usr/lib/debug/usr/sbin/asterisk - from package: asterisk-dbg

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

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