Another common point of confusion is the tab. While Steam Guard is for logging in, it is not where you redeem game keys. Always navigate to Account Details > Product Code to find the right screen.
The "Redeem a Steam Key" link sometimes disappears or errors out. If this happens, use Method 1 (mobile browser). steam redeem key mobile
Many users get lost in the Steam Mobile App because the "Redeem Code" option is hidden inside the "Account Details" menu rather than being on the main storefront. Another common point of confusion is the tab