Full basement and stem walls for garage

[JoAnn]

Me thinks you need to delete the entire foundation and rebuild it.

Is the garage designated as a 'garage' in the room dbx? If not, do that first.
Then Build / Floor / Foundation. When the foundation defaults box pops up, set your wall height to what you want.
Does that make a difference?

You will find information in your reference manual regarding 'Foundations / foundations and rooms'.