I have Microsoft balance but I can't buy anything, because I only have mobile.

Microsoft account balance from a gift card can only be spent in the Microsoft Store and other Participating Stores, not directly inside every mobile app or site.

Key points from the rules and support articles:

1. Where the balance can be used
   • Money redeemed from a Microsoft gift card is added to the Microsoft account and can be used to buy “movies, games, in-app purchases, and physical goods” from the Microsoft Store and other Participating Stores.
   • It can be used to purchase an Xbox-related subscription, but not every type of digital item or subscription is eligible. For example, it cannot be used for some in-app subscriptions or to obtain cash or cash substitutes.
   • The store and purchase options are designed around Microsoft Store on web, Windows, and Xbox. Mobile-only flows often do not expose all purchase options.
2. Platform/device limitation
   • The terms state that using the account balance “may require certain hardware or software specifications, including Internet access service.” In practice, this means some purchases (like certain game currencies or bundles) are only exposed through Xbox or Windows Store experiences.
   • If the Robux offer being targeted is only sold via Xbox or Windows Store, it will not be possible to complete that purchase using only a mobile device, even if there is Microsoft balance available.
3. What can be done with the current balance
   • The balance can still be used for other eligible products and content available in the Microsoft Store that can be purchased from a browser or app accessible on the mobile device (for example, some games, apps, movies, or Xbox subscriptions), as long as those items are allowed to be purchased with Microsoft account balance.
   • If a specific Robux/skin offer is only available via Xbox/Windows, there is no supported workaround to bypass that device requirement using only a phone.

In summary, the limitation being hit is by design: Microsoft account balance is restricted to certain stores and purchase types, and some of those require access via Xbox or Windows. The balance can still be used for other eligible items, but buying that specific Robux/skin offer may not be possible from mobile alone.

References:

• Add money to your Microsoft account
• Spend the money in my Microsoft account
• Microsoft gift cards terms and conditions