Hi there! I understand you're having trouble checking in via the Norwegian Travel Assistant app on your iPhone. Let's try to get this sorted out together.

To start off, could you please tell me a bit more about what happens when you try to check in? For example:
- Are you seeing any specific error messages?
- Does the app freeze, crash, or just not complete the check-in process?
- Have you been able to check in using the app before on this device?
- Also, have you tried checking in via the Norwegian website or another device?

Meanwhile, here are some initial steps you can try to resolve common app check-in issues on iOS 18.7:

1. Ensure the App is Updated
Go to the App Store and check if there's an update available for the Norwegian Travel Assistant app. Updates often fix bugs related to check-in functionalities.

2. Restart the App and Device
- Swipe up from the bottom and pause to open the app switcher, then swipe the Norwegian app off the screen to close it.
- Restart your iPhone by holding the side button and volume button until the slider appears. Slide to power off, then turn it back on.

3. Check Internet Connection
Make sure you have a stable internet connection (Wi-Fi or cellular). Sometimes check-in fails if the connection is interrupted.

4. Confirm Check-in Window
Norwegian typically allows check-in starting 48 hours before departure. If it's too early or too late, the option may not be available in the app5.

5. Clear App Cache / Reinstall App
Though iOS doesn’t have a direct cache-clear option, uninstalling and reinstalling the app can clear any corrupted data that might interfere with check-in.

6. Verify Details in the App
Double-check that your booking reference, name, and flight details are entered correctly. Sometimes discrepancies cause the check-in to fail.

If after these steps the problem persists or you get a specific error message, please share it with me so I can help dig deeper. Also, if you would like, I can guide you through alternative ways to check in or escalate to our technical team.

Does this sound good? Let me know where you are with these steps or if you need help with any of them!