I have got FS 2020 via the Xbox GamePass for PC.
Firstly it asked to download a 93GB update - which it eventually did.
But now when starting the game the yellow ‘fly’ button doesn’t do anything, I have selected a plane and start/end airports. If I go to flying lessons they are kinda ‘greyed out’, and I can’t click on them.
I have restarted various times - at one point it said ‘download content update - 3gb’, but then that got stuck for many minutes.
I have restarted again now and I still have the problem.
5 hours… 100GB+… and I’m still on the ground.
I really wanna play this game, but this is driving me nuts, it’s 5:11am!!!
I haven’t had that issue, but this may work for you. Hope you get in the air soon! 
This was a fix from @PaneledMantis30:
The downloader/installer can get stuck while decompressing downloaded files. If you have partially downloaded files, or compressed download files, the game might tell you it needs to update (but get stuck), or have unavailable options in-game, or fail to load all the way into the game.
Try this:
-Close the game
-Go to C:\Users[user]\AppData\Local\Packages\Microsoft.FlightSimulator_…
-Go to LocalCache\Packages\Official\OneStore
-Sort by Date modified and remove all FSPATCH files (they should be at the top)
-Launch the game (as administrator preferably)
This got my download unstuck and allowed installation to complete.
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I have had the exact same issue, I have been trying for nearly 11 hours and it does no seem to work, even the solution posted here.
Wow! 11 hours. ‘only’ 5 here now. I’ts now on 56GB out of 59GB for some more files. ‘fs-base-cgl-0.1.21.fspackage.008’ which is now decomrpessing. But I think I’ve been here before. Now 6:17am.
I have the same issue. I think that some of the content is missing but the Sim isn’t attempting to download what it doesn’t have (my install folder is only 48Gb, which seems too small?). A lot of times during the download the program kept closing itself down, the final time it really wreaked havoc on the system (even lost mouse connectivity) - guessing that was related.
I have submitted a support request on ZenDesk and will update here if I get anything back but am thinking it’s going to need to be reinstalled sadly.
Attention everyone with this issue, might be worth force close and restart, or even restart your machine and try again. I just picked up 43.25Gb of updates in Content Manager on startup, despite getting past this screen without needing updates nearly a dozen times before now in the last few hours.
Going on 12 hours of trying here and still stuck on it trying to download the same files over and over. I’ve tried reinstalling the game, rebooting a million times, deleting the fspatch files, nothing fixes it and I’m ready to just refund it at this point 