Error 12 is caused by your game hitting a memory wall, it will start to slow down and lag after some play time as cathyknits mentioned, the game won't have enough resources to use to save the game, however that is temporary, what I have tried and worked for me is to wait a while (stay calm and keep playing), reset your town "resetsim *" got into edit town mod and try there, this will reduce the resources that game uses, since most townies will be pushed to sleep, and then try again, should be working, also consider ditching any cc, or buying more memory