apt-add-repository: 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/bin/apt-add-repository - from package: software-properties-common

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

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