onsgmls: 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/onsgmls - from package: opensp

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

On Debian possible default full paths are:

/usr/bin/onsgmls - from package: opensp

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

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