add-shell: 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/add-shell - from package: debianutils

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