Make sure Streamer.bot is running and its WebSocket Server is started under Servers/Clients → WebSocket Server. The dock uses 127.0.0.1 and port 8080 by default.
If you use a different address, port or a password, add them to the dock URL, for example ?address=127.0.0.1&port=8080&password=YOUR-PASSWORD. Leave out the password part if you do not use one, and leave Enforce All Requests unchecked in Streamer.bot.
If the dock connects but says the handler is missing or disabled, check that you imported the extension and enabled its handler action. If it says Streamer.bot did not answer, check that the handler still has its Trigger Custom Event subaction after the code, with %sbClientResponse% as the event name and Use Args enabled.