"Error. Requires missing expansion pack"

I know this is along dead thread, but since I ran into the same problem today and this page came up in my search, I thought I'd share the fix that worked for me:

You need to make sure the box next to Pets in the "Expansions & Stuff" section is checked as Active. For some reason, my launcher unchecked all my expansions after I started downloading content, which resulted in that annoying error message when I tried to download the Pets items.