nslookup: 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/nslookup - from package: bind-utils

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

On Debian possible default full paths are:

/usr/bin/nslookup - from package: dnsutils

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

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