HOW TO CREATE APARTMENT?

@kwohld  Without mods, any given apartment building, or any lot in general, can only have one proper household.  If you'd like to add NPCs to the other area, so they come and go as if they lived there, you can do so.  First, make sure the door you want them to use is an NPC door, not a regular one.  Then click on that door and use NRaas MasterController's Add Sim command to add the homeless sim(s) of your choice.  Keep in mind that this would make residents homeless, so it's best to only add it for already-homeless sims like maids or bartenders.

If you want to assign an NPC to a door that faces outwards, not an interior door, you'll need Buzzler's NPC Door Fix, part of his "Builder Stuff" package hosted at Mod The Sims.

If you really want inactive residents on the lot, you'll need Ani's Apartment mod, hosted on the NRaas site (but developed by Ani, not Twallan).  I haven't used it, so if it's not self-explanatory, it would be best to ask for help on the NRaas site.

Whatever approach you choose, I'd suggest saving immediately before and being prepared to quit without saving if this doesn't work out the way you intended.  Sometimes it's easier to try again from the beginning than to try to what went wrong.

Thank you for your response PuzzlezAddict, I ended up installing Buzzler's Fix NPC Door mod. Do you happen to know much about this mod? I'm trying to add Olive Kipling as my neighbor, and after I click Nraas//Master Controller//Add Sim, nothing happens. It still says "nobody lives here". I'm getting so close, though 

You are a genius! So just to reiterate for any other players having this issue, after I downloaded Buzzler's Fix NPC Door mod at Mod The Sims and added my playable family, I set my NPC door and moved Olive Kipling in through MasterController's Add Sim function, then I reset Olive Kipling. It showed as "nobody home" when my Sim rang the bell, but after she buzzed them at the callbox, I am able to ring the doorbell and talk to Olive if she's at home. Thank you so very much PuzzlezAddict!!! 😃