Which way did you create the sim? Did you use the story mode? That has acted up before and not given enough funds to buy a lot. I haven't seen any other posts indicating any trouble with it for a long time but that's not to say it isn't happening for people, just that it hasn't been reported lately.
But in addition to what@SheriGRsaid, you can use a cheat to get a household onto a lot when the household doesn't have enough money to buy it. At the map screen, you can open the cheat window and use the "freerealestate on" cheat and then all the rectangles around the lots will turn green and you can place your household on any residential lot. Then you can use caching and motherlode to give them the amount of money you want them to have once you've got them placed.
Hope this helps.
