When you try to open the terminal from Cpanel, it shows you an error in the red screen like the following:
The WebSocket handshake failed at 1:34:27 PM.
The error is appearing because Cpanel uses a socket to create a terminal window for you from the Cpanel. But this verifies the origin of your URL. If you are behind a proxy, then Cpanel won’t let the socket establish.
Most of the time, I have seen the user using Cloudflare behind the domain and using the same domain to access the Cpanel. Unfortunately, Cloudflare’s proxied IP won’t be able to create the websocket. Hence, you must use a domain or hostname or the server IP to access the Cpanel, to use Terminal.