What causes PSN error WC-40377-1 when redeeming a wallet card?

Error WC-40377-1 (also E-82000138) means PlayStation failed to add funds to your wallet using a voucher or PSN card. The main causes are: your wallet balance is already at or near the $250 USD limit (spend some first then retry), a typo in the code (copy-paste instead of typing), an expired or not-yet-active code (check dates on the card or email), a region mismatch (PSN cards are region-locked — check Settings → Account Management → Country/Region), or the code was already used (check Transaction History in Account Management). If none apply, check the PSN Status page for outages.