If I put "sandwich" instead "a sandwich", it doesn't accept it...
A "бутерброд" is a “канапка“ in Ukrainian and it's not a sandwich. It's an open-faced slice of bread, that is a canapé, hence канапка! Ukrainians actually rarely eat sandwiches (two sliced of bread with something between them.
"Я їм канапку на сніданок." should be accepted also. (Never used 'бутерброд' in Ukrainian before.)
"I eat sandwich for breakfast." should be also viable, as a choice of cuisine.