khelpcenter: 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/khelpcenter - from package: kdebase-runtime

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

On Debian possible default full paths are:

/usr/bin/khelpcenter - from package: khelpcenter4

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

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