New XBOX S X new user can't sign in

I just bought a brand new Xbox One X, but when I go to the store, Flight Simulator and other Xbox Series X/S games say they are "not playable on this device" and I can't get Flight Simulator to install on my brand new Xbox One X because it says it's "not compatible". Very frustrating.

So you bought an Xbox One X and expected to be able to play an Xbox Series X game on it?

Why would a game made for an Xbox Series X work on an Xbox One X?

If you want to play Xbox Series X/S games I would take your brand new Xbox One X back to the store you bought it and see if they will take it back and you could try and buy an Xbox Series S (Series X consoles are very hard to find, but a Series S can be found fairly easilly)

Someone know if the new Xbox Controller S/X will be compatible with the old Xbox One ??

You're lucky. You can use the new Series X | S controllers on the old Xbox One.

There is going to be an Xbox One version of the game, but no news about an Xbox Series S/X version. Most likely it will come to the Series X/S as a backward compatible title. But as I say no news yet.

Xbox 360 games do not appear to see these users and I'm unable to select them when adding players to 360-games.

User accounts work differently on the 360 (and the 360 emulator on the Xbox One and Series X/S) to the way they do on Xbox One and Series X/S. The 360 had user profiles that needed to be downloaded first.

It's been a while so you'll need to bear with me. If remember you will need to sign in first with this other user you've created, then launch the 360 game. This will download the additional new profile to the console. I think, after this you should be able
to add this player to your game.

The Xbox X is better than the S so it depends on if you need the extra power the X is delivering otherwise getting a new S is a good idea in my opinion. (New warranty period and all)