I would like to suggest to add "Let's drink milk" as an available correct option, as it was taught in both ways (you and I, let's) in previous lessons.
I cannot pinpoint the lesson but do know it has been previously accepted both ways.
To be "Let's" you have to have E before the verb: E inu kāua i ka waiū.
why is 'we both drink milk' not accepted? Is 'you and I' not interchangeable with 'both'?
In Hawaiian there are 2 ways to say "we both": māua and kāua. A good English translation should include that level of specificity.
Ka:ua also means 'we both'.
Why am I wrong?
That's a good question. Did you take a screen shot so we can see what you wrote and help you figure it out?