If this notice is all you can see, the app is open in a preview window that cannot run web apps — this is what the iPhone Files app does with HTML.
On iPhone: a local file can never run here. The app must be served from a web address — see the included SETUP.md for a one-off, five-minute deploy. Then open that address in Safari and use Share → Add to Home Screen.
On Android: open this file from your Files app and choose Chrome. Then use ⋮ → Add to Home screen.
On a computer: double-click the file, or drag it into any browser window.
Your data is stored only in the browser you open it in. Nothing is uploaded and no account is needed.
To keep this app: tap Share below, then Add to Home Screen. It then works offline.