sgmlcheck: 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/sgmlcheck - from package: linuxdoc-tools

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

On Debian possible default full paths are:

/usr/bin/sgmlcheck - from package: linuxdoc-tools

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

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