Why does PlayStation say "voucher code is incorrect or no longer valid"?

This error has six main causes: a typo (watch for 0/O, 1/I, 2/Z — always copy-paste from email), wrong region (PSN codes are region-locked — check Settings → Account Management → Country/Region), expired code (promotional codes expire after 6–12 months), a physical card not activated by the retailer (return to store with receipt), the code being already used (check Transaction History in Account Management first), or being signed into the wrong PSN account. If none of these apply, check the PlayStation Network Status page for outages or try redeeming at store.playstation.com.