deja-dup: 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/deja-dup - from package: deja-dup

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

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