Can you buy V-Bucks on mobile?
Fortnite's mobile situation is particular. Due to the conflict between Epic Games and Apple/Google, Fortnite isn't in official app stores and buying V-Bucks on mobile has limitations.
Fortnite on iOS and Android
Fortnite was removed from the App Store and Google Play in 2020 following the dispute between Epic and Apple/Google. Currently you can play on iOS via streaming (GeForce Now or Xbox Cloud Gaming) and on Android by downloading the APK directly from Epic Games.
Can I buy V-Bucks from mobile?
Yes, but not directly in the app. The easiest solution is to buy V-Bucks on vbs.ar from your mobile browser. V-Bucks credit to your Epic Games account and are available on all platforms.
Why you can't buy in the iOS app
Apple blocks direct V-Bucks purchases in Fortnite because Epic implemented its own payment system without paying the 30% commission. Until the legal dispute resolves, in-app purchases aren't available on iOS.
The simplest solution
Buy V-Bucks on vbs.ar from any device (mobile or PC). No need to go through app stores. The process is secure and V-Bucks arrive in your account regardless of which platform you play on.