“SIPR” is an acronym that stands for “Secure Internet Protocol Router” and it just means that there is a type of internet just for secret stuff. The knowledge of this isn’t secret in and of itself (there is a Wikipedia page about “SIPRnet”) but access to it is held in strict confidence.
In fact, it is sometimes held in such confidence that even people who really have a valid reason to use it are sometimes denied access. They why of that I cannot say; perhaps because there is a bit of work that goes into validating someone’s need to access this “secret internet”. There are background investigations and such and maybe some agencies or units just don’t want to go through the headache. There may also be a budget angle to it; I can only presume that this is sort of expensive as well.
I’ve met people who worked in various parts of Intelligence that had “secret” clearance and the like but who never got access to the SIPR net (It is pronounced “sipper”, like sipping a drink). You have to get clearance and be given access by someone who is already in the network, and they themselves have to be authorized to hand out access– an insider can’t just give it to anyone.
Many military units are supposed to have a special, secure room where you can access this SIPR net. It’s kept locked and you can’t bring any other electronics in there like laptops, cell phones, even smart watches. Because of this, they are sometimes called “vaults”, or jokingly referred to as “dungeons”.
Having a SIPR vault that you can’t find or access is frustrating when someone comes to you needing a briefing on secret goings-on and you can’t do anything about it. Here, at least, Juni and Emma have at least found it, even though it took some Tomb Raider type action on their part.