trying to buy and install by using gift cards and parental supervision

Hello Olivier, my name is Jhon and it will be a pleasure to help you.

Check that your game purchase appears as "Completed" using the following link:

https://account.microsoft.com/billing/orders/

If everything appears in order, perform the following procedure in your console:

- First, make sure you sign in to the same Microsoft account you purchased the game with and not a different one.

1. In your console settings, go to the "Account" section and select the "Delete accounts" option and choose the account in question. This will delete your profile from the console but you won't lose any of your data, don't worry. .

2. With the Xbox on, press the power button on the front of the console for 10 seconds until it turns off completely.

3. Unplug the power cord from the console and wait 30 seconds.

4. Reconnect the power cable to your console and turn it on normally.

5. Then add the account back to your console so that all your profile information is downloaded again, it won't take long.

6. Check if the problem persists; If so, reset your console to its factory settings, keeping your games and apps as follows:

Go to Settings > System > Console Info > Reset Console > Reset and Keep Games and Apps.

This way your console will be reconfigured and you will have to add your account again. Games and apps won't be deleted or game progress will be lost when they're saved to the Xbox cloud.

7. Check if the game already appears in your library or to download from the Console Store.

Credits for this information go to advisor Gerardo since this response was based on said information:
https://answers.microsoft.com/thread/806e2146-01e2-4b4a-a187-67089fc8652f

I hope the information I gave you has been useful to you, have a happy day.
Jhon

Are you under parental control restrictions?

Have you redeemed your Gift Card on the store before trying to buy Fortnite's Currency?

then put him on a child account or supervise his use

child accounts are what parents should be using not full accounts

that's like giving your child your creditcard or debit card and your pin

Yep parentsal responcibility always lies with the parents not inanimate objects

That what your credit card company will tell you

Don't let inanimate objects be a babysitter

Parental supervision is always required

i think you should try to redeem that gift card somewhere else you can buy a xbox gift card, buy it, and use it here.