Gifts card can only be redeem in yor web browser not at roblox app.
the steps you must follow to redeem your gift card at Roblox.com/redeem:
1. Log into your account on a browser.
2.Go to the Gift Card Redemption Page.
3.Enter PIN Code then select "Redeem".
4.A success message will appear when you successfully add the Credit to your account.
5.Click Use Credit to use your credit. You can read the Using Gift Card Credit help article for help on the next steps or Click Close to close the message and redeem another gift card.