Do these:

• Try kill 1 in your repl's shell. This restarts the repl.
• Make sure the code is running while you open the webview URL.
• Make sure you have set the right internal port. If you want help reply with link to your replit code and I'll check it.
• Make sure your .dev URL has the port 3000. ([URL] :3000)
• Try the webview in the Replit interface. Click the new tab (+) button where it says the open files` names on the top of the interface and search for webview. If the webview in Replit interface works, then copy the link above the webview page and paste it into the address bar.