In the current state, it was only possible to resolve it by saving the game, closing, opening again and loading the game after opening it again, but when completing the contracts for the new game session, the problem occurs again.
Viewing post in C3 - Car Card Collector comments
I posted a couple of save bug logs in the Discord bug-reports section.
To repeat it, just start the game, try to save (a message saying it was not possible to save will appear) and it fails. Go to Dealership -> Restock -> Play -> Save and it will be saved normally.
In fact, this also happens when I collect without restocking the dealership and try to save.