When you complete the Witch Queen campaign without completing all missions on Legend difficulty on a single character, you will be offered a generic exotic engram you can redeem. If that is what you were offered, then one or more of the Witch Queen campaign missions was not completed. You can check which ones you need to complete in the Legendary mission selector on the Throne World map. Look through all the available missions and make sure that your Titan does not have the modifier "A Challenge Awaits..." active on any of them. If you find any with that modifier, you will need to complete those on Legendary difficulty. Once all have been completed on Legendary difficulty, Ikora will offer you either of the available exotics in a menu called A Legendary Hero. I've detailed this information in this image archive: https://imgur.com/a/qSopcMo