yelp-build: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/yelp-build - from package: yelp-tools

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