Can't Talk To Preston to Reclaim The Castle

[Nocture]

I'm at the point in Chapter 2 where you must recruit your allies to attack Gunner's Plaza. I haven't claimed The Castle yet and I want the Maximum amount of support from all my allies, but I can't talk to Preston without the "I need your help attacking The Gunners" dialogue appearing, and I haven't even started the Railroad questline yet, so that'll be interesting to say the least.

Any clue on what I can do?

 

[Cessori]

If you've met one of the SS2 Factions (CPD and Nightingale), you can increase your level of friendship with them while the main quest, War is Good for Business by doing their quests.

Gaining the level of friendship with the Vanilla Factions (BOS, Minutemen, Railroad, Institute) you want, should be completed before you start War is Good for Business.
If you have met the faction, you can complete a quest to gain their trust while War is Good for Business is in progress, but as soon as you gain that first level of friendship, that line "I need your help attacking the gunners" become available, and you are able to bring some of that faction to the fight.
The levels of friendship for the factions are related to completing their quests.

You could return to a save before starting War is Good for Business and get those vanilla quests done. The Wiki page linked previously has the requirements for Ally Level and the associated quests required.

 

[lonewolfcavalry]

taking independence (the castle quest) only triggers if you have a certain number of settlements claimed(not sure of the number)