"You are missing packages that are essential to run Microsoft Flight Simulator"

User Support Hub Install, Performance & Graphics
24

I think the easy way is to:

  1. Save your UserCfg.opt file. (or rename it to myUserCfg.opt)
    The location of your “Installed Packages” is on the last line of this file.

  2. Save any mods

  3. Delete the UserCfg.opt file.
    Run FS2020.
    This will make the FS2020 install run.
    It will download and run the 2 GB Launcher to your C: (or boot) drive.

  4. When the install screen displays,
    click the address box and tell it where your “Installed Packages” are.
    FS2020 will check and require you to download any folders/files that are corrupted or missing.

  5. Run FS2020 with the installed default UserCfg.opt.

  6. If okay, replace the default UserCfg.opt with your saved one.
    Replace your saved mods.