ast: 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
/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/lib/jruby/bin/ast - from package: jruby

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