add_user.inc: 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
/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/share/ampache/www/bin/install/add_user.inc - from package: ampache-common

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