Eat fruit should be accepted as fruit is both singular and plural.
'fruit' is correct in this context. 'fruits' is wrong. Only when used in an abstract sense is the plural used - e.g. 'the fruits of our labour'.
One can imagine contexts where "fruits" would be correct ("Try to eat more fruits and vegetables") but "fruit" is better here. "Fruits" means "kinds of fruit." Duolingo should accept both.