rbenv: 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/rbenv - from package: rbenv

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

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