mdoc-validate: 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-validate - from package: monodoc

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

On Debian possible default full paths are:

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

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

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