@PhoebeSnoww When you say you can't download from the Gallery, do you mean you can't place lots, or you can't even save them without placing them? If you just open the Gallery and click on an item, you should see a "save to my library" button in the lower-left corner even if you're not working on an individual lot; please let me know what happens when you click this.
Please also clarify what you mean when you say you can't start new saves with your own sims. Do you mean you can't import your sims into CAS, or something else? And if it's about importing the sims, are you trying to import them from the Gallery itself or from My Library? Please let me know whether the latter works even if the former does not.