Ronnie.

[Blinkybubs]

Ronnie isn't showing up at the castle, is there something I need to do for her to come? or is there a way to use the inaccessible terminal myself?

[The Inept European]

I am guessing you are in the middle of the quest to get the Castle? Old Guns specifically.

You have to actually leave for a while and come back for Ronnie arrives and Old Guns starts. And she is the only way to unlock that door.

[Liquid Inc]

and one of the few NPC's i want to shoot the moment she turns up

[Ommamar]

If you listen to freedom radio you will get a broadcast about coming back to the Castle for an unknown issue. If you don't care for that good ol Preston will be glad to tell you about it when he sees you. I Agree with Liquid Inc I didn't much care for Ronnie, she isn't bland so I will give Bethesda props for writing her in away I actually care about how she acts.

[The Inept European]

Yes not my cup of tea either but I appreciate that they wrote her with some backbone.

[Xenon The Noble]

I always got the start of the quest over the radio. I've only done it three times, tho. Did you complete the castle quest?

[Liquid Inc]

yeah, not bland, and has a backbone but still.. She's at best a traitor, and worst a deserter who wants to come to her old position, and still treats you like dirt

ugh, i might just shoot her this time and see what happens, but she's probably essential

[Xenon The Noble]

Originally posted by Liquid Inc:

yeah, not bland, and has a backbone but still.. She's at best a traitor, and worst a deserter who wants to come to her old position, and still treats you like dirt

ugh, i might just shoot her this time and see what happens, but she's probably essential

She's essential. And if you walk away, she follow you for a good while out of the Castle.
But bad things happen and I'm not sure why OP can't get the "old guns" quest to progress. Not enough settlements? I think that requirement is tied to the retake the castle quest.
https://fallout.fandom.com/wiki/Old_Guns#Bugs

[The Inept European]

I think OP is expecting Old Guns to run immediately after claiming the Castle but that's not how it works, you have to leave and come back when Ronnie is there.