load_gen: 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/lib/zookeeper/bin/load_gen - from package: zookeeper-bin

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