knsupdate: 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/knsupdate - from package: knot

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

On Debian possible default full paths are:

/usr/bin/knsupdate - from package: knot-dnsutils

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

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