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You should be able to move your homeless Sim onto an empty lot -
Select Options in the lower left corner > Edit Town > Change Active Household
;
> Choose Household (pick a household from the town and select them to play)
;
or Move In Household (select a household from the household library and place them on an empty lot.)
Pick Family from Library, Choose the lot, Purchase the lot, Place them on an empty lot and then start playing the family.
Does either method not work for you? Is that's what happening in your game?