"Kamu makan apel, saya makan jeruk."

Translation: You eat an apple, I eat an orange.

[SalaiKheng]

I answered " you eat apples, I eat oranges."
It said I was wrong. there weren't any explanation about how and where do i use singular and plural.

[ALLintolearning3]

It depends on the exercise you had. Which instructions did Duolingo give you? Both singular or plural should be accepted. Sometimes we translate, but other times you are supposed to put the original sentence.

[olyglotED]

So these food objects could be plural or singular depending on context? I understand that when "sebuah" (one/a/an) is written it eliminates the ambiguity...

[Rick392366]

So these food objects could be plural or singular depending on context?

Yes, that's right.
It depends on the context.

I understand that when "sebuah" (one/a/an) is written it eliminates the ambiguity...

Yes, that's also correct.
When "sebuah" (or another counter/classifier) is used, then it should be obvious if it's singular/plural.

For example :
Kamu makan sebuah apel = You eat an apple.
Kamu makan dua apel = You eat two apples.
Kamu makan banyak apel = You eat many apples.

[Dosoon79]

Just to add on. For fruits, the counter word usually used is "sebiji". Sebuah is usually used for big things for example house, country.

Sebuah rumah = a house
Sebuah negara = a country