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

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