"You have a bottle with milk."
Translation:Voi aveți o sticlă de lapte.
Normally both singular and plural is accepted in an answer to an exercise with “you.” That the suggested answer was in plural does not mean that you didn't have a quite different mistake in your answer – Duolingo tries to suggest an answer as close as possible to yours but does not always succeed.
So the normal thing to do would be to “⚐ report” your answer (“my answer should have been accepted”). If you want to complain here, be sure to include the full text of your answer; otherwise it's impossible to tell what happened.