I cannot purchase any V-Bucks from the in-game store or the Microsoft Store

V-Bucks can be purchased in Fortnite which you can write a bug report if it isn't working , I usually reload the game if it isn't working, it has happened to me a few times,

If you live in Belgium you might just cant buy ingame coins for any game..

Same with fifa

You have purchased V-Bucks for the Xbox version. You should have purchased them for the PC directly from Epic Games, or from a local or online game store.

This is taken directly from the FAQ on Epic Games site

Can I use these V-Bucks across devices?

V-Bucks purchased on Xbox, PlayStation or Switch cannot be spent on other devices. Any Fortnite content you buy with your V-Bucks will be available on every device linked to your Fortnite account, regardless of which device the content was bought on.

They did say that they purchased them from the Microsoft store not from the Xbox. The link that they shared to the Microsoft store shows in the listing that they can be used on PC.

You have purchased V-Bucks for the Xbox version. You should have purchased them for the PC directly from Epic Games, or from a local or online game store.

This is taken directly from the FAQ on Epic Games site

Can I use these V-Bucks across devices?

V-Bucks purchased on Xbox, PlayStation or Switch cannot be spent on other devices. Any Fortnite content you buy with your V-Bucks will be available on every device linked to your Fortnite account, regardless of which device the content was bought on.

Hey Pastoral,

Normally, you are contacted whenever a purchase is done through the Microsoft store. In this case however, V-Bucks are purchased from the in game store which bypasses our system which notifies you of the purchase. I hope this clears out any confusion.