I can't add a flight to Calendar

If you encounter any challenges with third-party applications, the best course of action is to directly contact the app developer for further assistance. Here's a brief guide on how to do it:

1. Identify the app developer.
2. Visit their website or app store listing.
3. Look for their contact information or support channels.
4. Clearly describe the issue you are facing.
5. Provide any necessary information they may need.
6. Follow their instructions and provide feedback.
7. Be patient and follow up if needed.

By reaching out to the app developer, you increase your chances of receiving accurate and personalized support for the challenges you're experiencing. To contact an App developer --> How to contact an app developer - Apple Support

Where are calendar events stored? Not on iCloud?

You have not mentioned where your calendar events are? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

Refer to the pic below, you may get your answer

Which is your default calendar? Settings > Calendar > Default Calendar

Settings > Calendar > Sync > [All Events] - Watch the GIF below

Do you have something similar to quantas?

To import your flights to your desktop calendar application, open your calendar and click Import iCalendar (.ics). Browse to locate the flight_calendar.ics file ...

Adding your booking to Calendar

LOT says: With the help of our application, you are able to not only efficiently search and book flights, but also manage your reservations. Create an account and add ...

Discover the benefits of the LOT mobile app! | LOT.com

JackHops wrote:

And "maybe" the newest iOS 17.4 issued uyesterday evening will help?

Please share your tips and advices.

Check what iOS 17.4 has to offer. --> About iOS 17 Updates - Apple Support (IN)