nsgmls: 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/nsgmls - from package: opensp

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

On Debian possible default full paths are:

/usr/bin/nsgmls - from package: sp

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

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