Issue description: Pokémon Trainer Club (PTC) accounts are not able to access the Pokémon GO Webstore, including the Offer Code Redemption page. This also affects PTC child accounts.
Issue status: Investigating.
Workaround: In the meantime, you have two options:
Link another login provider to your account: This will allow you to redeem offer codes using the linked account.
Contact us via in-game support: Our team is happy to help you to find a solution.