If you want to "give V-bucks", you have to give a gift card for the platform that your giftee is playing on. So if they play on Xbox One, give them a gift card for Xbox One. If they play on Switch ...
For many, you'll unlock this naturally if you're an avid buyer of V-Bucks gift cards. Here is how you can cancel a Fortnite Crew subscription on all platforms. Like most "freebies" that Fortnite ...