gnome-boxes: 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/gnome-boxes - from package: gnome-boxes

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

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