irb2.1: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/irb2.1 - from package: ruby2.1

You may install the required package with command apt-get install {package}
Error in other languages:
irb2.1: bevel nie gevind nie
irb2.1: comandă negăsită
irb2.1: comando não encontrado
irb2.1: commande introuvable
irb2.1: command not found
irb2.1: käsku ei ole
irb2.1: không tìm thấy lệnh
irb2.1: komanda nerasta
irb2.1: Kommando nicht gefunden.
irb2.1: kommandot finns inte
irb2.1: komut yok
irb2.1: nie znaleziono polecenia
irb2.1: níor aimsíodh an t-ordú
irb2.1: no se encontró la orden
irb2.1: no s'ha trobat l'ordre
irb2.1: opdracht niet gevonden
irb2.1: parancs nem található
irb2.1: perintah tidak ditemukan
irb2.1: príkaz nenájdený
irb2.1: příkaz nenalezen
irb2.1: команда не найдена
irb2.1: командата не е открита
irb2.1: 命令找不到
irb2.1: コマンドが見つかりません