Flatmap in java8 Stream cannot be used with reduce?

set of Stream stream operations using Java8

 List<String> fruitList=Stream
                .of("apple","banana","coconut","Damson","Filbert","Lemon")
                .collect(Collectors.toList());

        List<String> meatList=Stream
                .of("beef","pork","chicken","lamp","oyster","salmon","tuna")
                .collect(Collectors.toList());


        List<List<?>> bigList=Stream.of(fruitList,meatList).collect(Collectors.toList());

I want to flatten the collection using the flatMap method and then use the longest string of the reduce specification

 bigList.stream().flatMap(x->x.stream()
                .reduce((String x,String y)->x.length()>y.length()?x:y)
                .ifPresent(System.out::println);

result error

error message is shown in figure

what is this error capture of? I haven"t seen any Stream article about

if you change the code to

 bigList.stream().flatMap(x->x.stream())
                .map(x->x+"")
                .reduce((String x,String y)->x.length()>y.length()?x:y)
                .ifPresent(System.out::println);

that"s fine

bigList.stream().flatMap(x->x.stream()).forEach(System.out::println)

this line of code also works normally.
all the elements after merging the two collections in turn bigList.stream (). FlatMap (x-> x.stream ()) should return
the collection of all elements

Why can"t I use the reduce method directly later?