The D-Key was killed by and later models’ bootrom security.
Ensure both files have the exact same name before the extension.
A "DKey" (sometimes called Device Key or Decryption Key) in the PS3 context refers to cryptographic keys used by the PlayStation 3 to encrypt and authenticate system firmware, game content, or device-specific data. These keys are part of the console’s security architecture; they allow the PS3 to verify and decrypt signed software, protecting Sony’s intellectual property and preventing tampering.
Unlike "jailbreak folder" games, Redump ISOs are 1:1 copies of retail discs and are encrypted. You cannot play them without the matching .dkey.