helpviewer: 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/helpviewer - from package: wxPython

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

On Debian possible default full paths are:

/usr/bin/helpviewer - from package: helpviewer.app
/usr/bin/helpviewer - from package: python-wxtools

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

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