libwebsockets-test-server-extpoll: 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 Debian possible default full paths are:

/usr/bin/libwebsockets-test-server-extpoll - from package: libwebsockets-test-server

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

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