California FTB is demanding I submit tax return that I already submitted.

I have been receiving demands by the California FTB to submit my 2015 CA tax return, which as far as I know was filed years ago now through TurboTax?

I have a single member LLC and it was my understanding that it was a passthrough as in I don't have to file two separate tax returns for myself and then for the LLC...?

I sent my tax return in and receipt from TurboTax that year etc and they continue to send demands for the CA tax return for my LLC that according to them owes $0.00...not sure what to do? I think I did actually owe some super small amount like $20 or so to the state that year but I'm pretty sure it would've been paid right then?

It also says on the notice:

"**if we show a zero balance ($0.00) we may have not received state tax returns from you and do not know if your entity is classified for tax purposes as a partnership, a corporation or a disregarded entity. After you file the missing tax returns, you may receive a balance due notice."

I'm guessing somehow Turbotax never submitted my 2015 taxes to California? Or do I have to submit a whole separate tax return even though I'm only a single member LLC/passthrough entity?

How would I go about doing anything for a years old tax return?

Any help is GREATLY APPRECIATED!