gnome-help: 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/gnome-help - from package: yelp

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

On Debian possible default full paths are:

/usr/bin/gnome-help - from package: yelp

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

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