Minecraft Launcher is currently not available in your account. Make sure you are signed in to the Store and try again. Here’s the error code, in case you need it: 0x803F8001

I had this same issue and it turned out that in the Microsoft Store there is a "play" button and a "get" button. If you press the "play" button before the "get" button it will give you an error. I have no idea why this was designed this way but it's definitely not ideal. Hopefully this solves your issue.

Edit: I should rephrase this as I realize now that it may be confusing, the thing that I just explained is how you get the minecraft launcher without the error. To get the windows update you should now be able to download 21H2 as an optional update. It should show up in a little box right below where a normal update would be in the settings.

When i try to launch Minecraft Launcher i get error code 0x803F8001 - Microsoft Community

look at joels solution. worked for me