TestFlight download link returning HTTP error 404?
That link is working for me so either your network is not routing enough traffic correctly to the server or the farm of servers Apple operates to run the store has one endpoint that's failing or down.

If you try again in 30 minutes or try from a different network (LTE / different carrier / different WiFi network) that might help you isolate if the network error is close to your device.

You also would want to do the usual things to rule out software or run time corruption on your iOS device. Restart the device - test other store links - sign out and back in to your App Store on a known good network.

In the end - you'd contact Apple online store support after you're sure it's not your device.