I have the same problem

I'm having the same problem :(
It was an option before and now that I found someone I want to move in... I can't figure out how to do it!
Silly old fashioned me - I just want them to be in the same household before they have a baby LOL

Mine doesn’t allow me the option because there is no space in the household... what can I do

I have the same exact problem. I’m on level 10 and I’m married and I still can’t move my husband in. I have another room mate and I’m trying to get them to be good friends so I can move him in that way

Get them married or proposed
They move in automatically

You need that player to give up that simple. If they are willing and you have a free slot to add another SIM then it can be completed.

Basically you need control of two devices or build a casualty for that other human to take their SIM.