Hey there. I am not sure why it links you to the Lightfall purchase, but it's also telling you that the pattern is not yet unlocked. if you go to 'Journey', then 'Patterns and Catalysts', you can see the unlock requirements for each weapon, and your progress towards them. Perpetualis requires 5x extractions, you're only on 2/5 so should not expect to see the weapon unlocked for crafting yet. Once you're 5/5, it'll let you craft it and not open the Lightfall ad page when you try.