mdoc-export-html: 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-export-html - from package: monodoc

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

On Debian possible default full paths are:

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

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

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