THE GREAT SOLUTION
I Found the problem of Default Contacts in Samsung S10, SM-973.
Was the SECURE FOLDER, cause inside have the Contacts too, but is not possible to remove or add gmail as Default.
SOLUTION: uninstall the secure Folder
In the SECURE FOLDER > SETTING > MORE SETTINGS > UNINSTALL.
I did it and solved the fu*@a problem.
Perfect! Thanks a lot.
In my case, all contacts that I tried to add from WhatsApp (shared by someone) were automatically sent to my SIM CARD, and, since the memory capacity of SIM CARD is limited, only part of the data was accepted to record.
Now, given that I have selected my Google account as default, the issue is over.