Hola Catriona! Both answers in Spanish are correct, that is to say, you can use the indefinite article or omit it. Without the article, it is more general, and with the article is a bit more specific (some, a few...). The reason why we translated it as "buy some beers for tonight" in English is because in this language it would sound a bit odd to say "buy beers", but in Spanish both options are acceptable. Thanks for your contribution! Silvia.