Flight Simulator 2020 - CD needed in order to start the game

This is a very simple and basic type of DRM (Digital Rights Management), which prevents people from buying a game, installing it, selling the discs again, and playing the game while no longer owning the discs.

It used to be very common in CD/DVD-based games in the 90s and 2000s, but went mostly extinct in the age of digital-only PC games (Steam). It is still in use in modern gaming consoles for the same reason.

If you wish to play the game without a disk, you must download and install the Digital Ownership File.

• Microsoft Flight Simulator
  • [...]
  • Known Issue: Some users seeing a "Please insert the Microsoft Flight Simulator disc" error when launching the game on Windows 10.
  • Workaround: In order to play the game digitally on Windows 10, you will need to have the Digital Ownership File installed as it is required to grant the necessary permissions to play. If you already have this file installed and are running in to this error, please uninstall and reinstall this file only and this should resolve the issue. There is no need to uninstall the entire 127+GB base game files. Uninstalling this small 10MB file is all that will be needed.

Source

Since the Digital Ownership File can not be purchased, your game should've shipped with a download code. If you bought the game second-hand, you're out of luck and will need to buy a new copy of the game, or a digital copy.