ruby2.1: 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/ruby2.1 - from package: ruby2.1
/usr/lib/ruby-standalone/bin/ruby2.1 - from package: ruby-standalone

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

Error in other languages:
ruby2.1: bevel nie gevind nie
ruby2.1: comandă negăsită
ruby2.1: comando não encontrado
ruby2.1: commande introuvable
ruby2.1: command not found
ruby2.1: käsku ei ole
ruby2.1: không tìm thấy lệnh
ruby2.1: komanda nerasta
ruby2.1: Kommando nicht gefunden.
ruby2.1: kommandot finns inte
ruby2.1: komut yok
ruby2.1: nie znaleziono polecenia
ruby2.1: níor aimsíodh an t-ordú
ruby2.1: no se encontró la orden
ruby2.1: no s'ha trobat l'ordre
ruby2.1: opdracht niet gevonden
ruby2.1: parancs nem található
ruby2.1: perintah tidak ditemukan
ruby2.1: príkaz nenájdený
ruby2.1: příkaz nenalezen
ruby2.1: команда не найдена
ruby2.1: командата не е открита
ruby2.1: 命令找不到
ruby2.1: コマンドが見つかりません