The Worker is running, but it can't find the dashboard files. This happens when the code was uploaded by hand instead of deployed from the repository.
Fix: deploy with the Deploy to Cloudflare button (or connect the GitHub repo in Workers & Pages → your Worker → Settings → Build). That bundles the dashboard (the ASSETS binding) and creates the KV namespace automatically.
Already have a separate dashboard site? Set a Worker variable PAGES_URL to its URL.