Hello, good day and thanks for your question. Sorry to know about this and I understand that you redeemed the Minecoins for Minecraft but it has not reflected on your account.
In this scenario, you will not be able to redeem the gift card again as it will prompt that it has been redeemed and not unless you might have redeemed it on a different Microsoft account instead of the intended account.
If you just redeemed it, it is best to wait it out for at least 24 hours and then logout and login again to the game and check it again and if it still does not show up then you can reach out to the Minecraft team at https://aka.ms/Minecraft-Support/ for this to be checked further.