The first thing to try is to delete the app from your phone and re-download it from the AppStore. If it still won't start, try the instructions below.
The Trailforks app will not work if you have iOS Restrictions feature enabled with websites restricted for only "Specific Websites Only".
You can either allow All Websites or add http://localhost to the allowed list, if you have the "Specific Websites Only" option selected.
The Trailforks app is a mix between a hybrid and native app, the Hybrid parts use a webview via Cordova, which is served via the http://localhost.