yelp: 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/yelp - from package: yelp

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

On Debian possible default full paths are:

/usr/bin/yelp - from package: yelp

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

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