If you've never unlocked the Recaster on any character you should be able to by progressing the Wayfinder's Voyage quest. However, if you previously unlocked the Recaster on a character, your alts or new characters won't be able to pick up Decrypting the Darkness. This is a known issue Bungie is investigating. https://www.bungie.net/en/Forums/Post/259485383?sort=0&page=0 [quote]The Decrypting the Darkness quest cannot be obtained from the Umbral Decoder, and access to it and the Prismatic Recaster cannot be unlocked.[/quote]