Hello everyone,
I had the same problem. I solved it with the steps below.
First, I disabled XMP in the BIOS so that the memory would work with the default frequency of 2400MHz.
Second, I uninstalled Microsoft Flight Simulator 2020 and deleted the folder with the files were it was instaled.
Third, install the game again.
It will work normally.
After the installation, you can enable XMP again on BIOS.
If that doesn’t work, try installing it with one memory module in your device.
Hope this helps.