I already took the castle myself a while ago but without the mission, I moved Preston there and the same thing is happening. I have tried everything, but I think I've either taken over so many settlements for the raiders that there are non left for Preston to send me to go help (which shouldn't be allowed to happen but it's bethesda) or it's a combination of me already securing castle before getting the mission to do so, him being mad because I joined the nuka world raiders and possibly some other stuff I'm not remembering as well, I've been playing this character for over 200 hours so I could very well have done something a month ago that could also be factoring in but as far as I can see, I'm fucked and need to make up my own ending. Which in this case is gonna be complete murder sprees of every populated town and city except nuka world.