mono: 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 CentOS possible default full paths are:

/usr/bin/mono - from package: mono-core

You may install the required package with command yum install {package}

On Debian possible default full paths are:

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

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

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