Unfortunately, the state is correct. At this point your only options are to install an original (not aftermarket) catalytic converter on your 49-state emission legal 1999 Toyota Avalon, which can only be obtained from the dealer. The California CAT won't work because of your Avalon exhaust configuration, and the 49-State legal CAT can not be shipped to California.
With that being said... the second option you have is to drive your Toyota Avalon to the nearest auto repair shop outside of California and have the catalytic converter replaced. It's the dealership alternative without breaking any California smog laws (in terms of having a 49-state emission CAT somehow shipped to CA, or purchased illegally through a muffler shop)... neither of which we recommend.