I had to restart three times. Just be meticulous and keep careful track of your letters. With code variations, the grid doesn’t give you a ton of feedback (and a couple of the clues are particularly tricky). The fill is challenging even once you know all the answers - it’s easy to make mistakes if your logic is not solid. However, I encourage you to persevere; it’s among the top 4-5 puzzles I’ve ever done. Final step is incredible.