mono-sgen: 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/bin/mono-sgen - from package: mono-runtime-sgen

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