ri2.1: 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/ri2.1 - from package: ruby2.1

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

Error in other languages:
ri2.1: bevel nie gevind nie
ri2.1: comandă negăsită
ri2.1: comando não encontrado
ri2.1: commande introuvable
ri2.1: command not found
ri2.1: käsku ei ole
ri2.1: không tìm thấy lệnh
ri2.1: komanda nerasta
ri2.1: Kommando nicht gefunden.
ri2.1: kommandot finns inte
ri2.1: komut yok
ri2.1: nie znaleziono polecenia
ri2.1: níor aimsíodh an t-ordú
ri2.1: no se encontró la orden
ri2.1: no s'ha trobat l'ordre
ri2.1: opdracht niet gevonden
ri2.1: parancs nem található
ri2.1: perintah tidak ditemukan
ri2.1: príkaz nenájdený
ri2.1: příkaz nenalezen
ri2.1: команда не найдена
ri2.1: командата не е открита
ri2.1: 命令找不到
ri2.1: コマンドが見つかりません