To whom it may concern, I jumped the gun in my previous post, and would ask you not to accept it as it is. I tried to add an other non-CGSize property to my struct with @Model applied, and I got the same error. It appears that adding one more property to my struct, of any type, may be triggering the error. Bottom line, if I find the time, I will try to form a minimal pared down version of my application, in which I can show you a single line of code change which triggers the error. Until I do so, I do not think my post would be useful to this group.

This is my first post on the Developers Forum, so please excuse any procedural errors on my part. I just ran into the "Failed to identify a store that can hold instances of" fatal error. (XCode Version 16.1 beta 2 (16B5014f), MacOS 15.4.1 (24E263)) In my case, (without presenting all of my code,) my code ran before I added a CGSize property to the class on which I'd applied @Model within my document-based MacOS application. Then after adding the code and during what appeared to be the first autosave, my application generated the fatal error already mentioned within UsesSwiftData2App. This is not a solution, just a piece of information which may or may not help while debugging this problem.