software-properties-gtk: 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/software-properties-gtk - from package: software-properties-gtk

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

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