How to backup games which are installed from Microsoft Store.?

you cannot access game files for windows store or xbox app, they are under an encrypted folder.

you can sign in on the new computer and download again and if it has cloud saves that will follow.

if it does not have cloud saves you can possibly transfer the saves using gamesave-manager which is a third party free utility that finds all games saves on a pc and stores them for you to move to a new system.

I don't understand how you're trying to install Microsoft store from the Microsoft store. If you already have the Microsoft store, then you should be able to install games on your PC. Or do you mean the Xbox App?

The Microsoft Store only provides an "Install from Xbox App" button for Xbox Game Pass games instead of a download option, which then opens a game's install page in the Xbox App.

I have this same problem for 2 weeks, and finally I found what works for me.

It isn't uninstall and install, run as adminstrator, graphics card problem etc. It is just a windows account problem.

Upon buying the game from Microsoft Store, I could not install the game as Microsoft Store says it could not find any devices to install into. Out of desperation to install the game, I created an account in my desktop (the one which requires you to log
in everytime you start the computer), and finally I was able to install. Then that's when the problem came, it just crashed to desktop after I press any key in the "Press Any Key to Start" screen.

After trying all the online suggestions, like install language pack, update drivers, disable certain applications, it still doesn't work. And tried of logging in everytime I turn on my desktop, I revert back to the stage before I purchase the game, using
my backup windows system image. After that I tried to install, and voila, it works!

So in summary, do not create an account in windows which requires you to log in. For my case, from a fresh window, install the xbox app, sign in, see the game you purchased, and install from there.

In Steam you can backup a game on an external HDD and then install it on a different computer, which is something Microsoft should implement in the Xbox(beta) application. Or something similar like that. You can also move Origin games to a HDD and then
just put it on a different computer and verify the files. You can also do that with games from the Epic Games Launcher.