ruby-build: 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/ruby-build - from package: ruby-build

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

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