By the comments to the question I am assuming this is a browser project. And by the error message, that it is shadow-cljs.
With shadow-cljs Calva opens up the REPL window a bit prematurely, because we still lack a reliable way to ensure the connection to the ClojureScript application. Once you have the prompt, like you have there (embarrassingly undefined), try reloading the web page with the application. You should be able to evaluate something then, and the prompt should get something less undefined.
I can recommend joining the #calva channel on the Clojurians Slack, for faster help.