There are a few ways you can snap out of an error 12, but unfortunately, if they don't work, then you'll have to give up and forfeit the progress you've made. Try pausing the game, then go into map view, and save from there. Make sure your game is paused when you try to save. If it doesn't work, zoom back into the household, unpause, pause again, try saving.
It's a little fiddly, but keep going between map-view and zoomed in while paused and you might get it to go, there's very few other ways to save past an error 12. The only way to prevent those from happening in the future is by Saving-As and giving the file a new name, also try to avoid using spaces in that name (as I've read those can lead to saving troubles in the future).
Save often, Save As often and back up your Sims in case you're just stuck.
If you simply can't save, another thing you can do is go into Edit Town mode and save your family to the Sim Bin. Messier, but you can put them back down in a new town with all the skill progress retained.