ns-activate: 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 Debian possible default full paths are:

/usr/sbin/ns-activate - from package: 389-ds-base

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

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