"You drink alcohol."
Translation:Tu bois de l'alcool.
Yes, alcool is a masculine noun and typically the partitive article for masculine nouns is de + le = du, but in this case it also begins with a vowel. Whenever a word begins with a vowel, it keeps the l' in front of the noun. de + l'+ word beginning in a vowel (feminine or masculine).
Hmm. If you wrote the answer correctly without typos, and it still counted it as wrong, you may have encountered a bug. Could you please post a screenshot if it happens again?
To post a screenshot here, you'll have to upload it to a hosting site like Imgur. Then right click on the picture and click "Copy image address" (or the equivalent in your browser). Then, here, type !(Pasted image address), replacing "Pasted image address" with what you just copied. Make sure not to leave any spaces.
"Vous buvez de l'alcool" is also correct."
For the purposes of Duolingo either answer is accepted provided that the verb is conjugated correctly. In real life, use "tu" with family, friends, children, and close colleagues. Use "vous" with strangers, your boss, and anyone to whom you owe respect, as well as with groups of people.