Hello Larry. While I am not nearly an expert at CA, interestingly, I too have been struggling with adding a garage today to a home with an existing basement and having all kinds of anomalies with foundation portions of the Mono Slab coming into the adjacent basement. So, the garage thing was fresh on my mind...
I do know that when adding a garage, there is no need to build a foundation separately if the garage will be on a monolithic slab as you had in your plan.
I took your plan removed the foundation on level 0 and then on level 1 added the foundation information to the garage in the room specification/structure. While I'm not sure what that message means either, it will no longer be there with level 0 removed.
I've added the plan and shots back to show you how I manage to build my garages in CA.
I know that this does not necessarily answer your question as to the meaning of the message, but I hope this helps you with moving forward, I know what spending all day on something to no avail is like...
Cheers...
Oh, and, providing my response to your question just gave me the answer to my all day frustration... I solved my issue! Thanks!
vary heights 1.plan