Most often, it's a $0 balance "Invalid" almost always means the card has already been used and now has a $0 balance. Keep in mind your Wallet shows a card's original value and never updates as you spend — so a balance showing there doesn't mean the card still has funds. Check the card's real balance first — see [How to check your gift card balance].
Other common causes
The code was entered by hand. Copy it straight from your Wallet rather than typing it — a single wrong character will read as invalid. Use the copy icon in the app to copy the full code.
It's in the wrong field. Some retailers have a separate "gift card" field and "promo code" field. Make sure your code is in the gift card field.
Still not working?
First confirm the card's real balance — see [How to check your gift card balance]. Remember the number in your Wallet is the original value, not what's left. If the confirmed balance still shows funds but the code is declined, email [[email protected]] with a screenshot and the last four digits of the card code.