Hey, This mission should be relatively easy and looking through everything on the internet says that I should have a letter in my inventory by investigating a library, There is no letter in my inventory and any attempt to forfeit the mission and go back does not fix it, this mission should not be a inventory grab to activate another mission, just activate it on the bookshelves or something, because I literally cannot progress the main story due to this bug and I want to keep playing. I searched through every archive in the room and even attained the information of each of the readable shelves. I do not have a letter, and cannot get the letter. please help!
The quest only had the actual letter in 2016, afaik that has since been changed to a simple knowledge entry. Check if you have the knowledge "Secret Letter from the Dark Chaser" in your knowledge UI (H).
Did you interact with the right book? As shown here in the video: https://youtu.be/GmdjxgYjImw?si=iusDKmyjcZDYgpDb&t=45
If you didnt get the second option, you might have to do some other quests to unlock it first.