This sample code for Landmark is really buggy, or at least not consistent with the present bersion of SwiftUI.
The best is to ignore it until it is stabilized.
I have the same issue. Even I downloaded the sample and compare between "Start point" and "Complete" files.
I copied exactly code but seems doesn't work either.
Better to wait for the final release.
Is the project folder on your Desktop or in a directory on your desktop?
I was getting the same error. Did a command + shift + k, closed the project, and moved it to the root of my home folder. Started back up and it worked fine.
I suspect it has sometime to do with accessing files on the Desktop, a "protected" directory in Catalina.
Make sure you're not running Xcode from your Downloads folder. Seems to cause issues like this for me.