Why does Steam say "Invalid Product Code"?

This error ("Invalid Product Code — The product code you've entered is not valid") has four main causes: a typo in the code (watch out for I/L/1 and O/0 mix-ups — always copy-paste from email instead of typing), entering the key in the wrong field (game keys go through Games → Activate a Product on Steam, not the wallet code field), the key being for a different platform like EA App or Epic Games, or the key being already used or fake. If the key is valid and for Steam, try activating at store.steampowered.com/account/registerkey. If it still fails, contact the seller with your order confirmation.