RESPONDER [starting point ] all issues

in the RESPONDER challenge , the unika.htb is not responding , even after adding it to /etc/hosts what might be the issue ?

I had the same issue until I realized I had to put my ip (found with ifconfig on the interface tun0 or whatever interface you have for openvpn) in the url. I suggest you to watch the walkthrough carefully if you have any issue, they literally write anything you need to know.