register-ds-admin: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/register-ds-admin - from package: 389-admin

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