mdoc: 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/mdoc - from package: monodoc

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

On Debian possible default full paths are:

/usr/bin/mdoc - from package: monodoc-base

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

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