If you've owned your iPhone SE for awhile there's a good chance you've accumulated a ton of apps and services. Most of those apps will work with iOS 11 and iOS 11.4.1, but there's a chance some of your older apps won't.
iOS 11 kills support for 32-bit apps. While most developers have updated their apps to support 64-bit, some haven't. Several popular apps, including Flappy Bird, won't work once you download the iOS 11 update to your device.
There's nothing you can do to fix this. It requires an update from the developer and apps like Flappy Bird (which isn't even on the App Store) probably won't get the necessary support.
If you want to look at which apps no longer work on your iPhone SE open up your Settings app. From there, tap on General, About, and then tap Applications.
If you're already on iOS 11 and you have a long list of apps that don't work, you're stuck until the developer decides to upgrade the app.