"Kateřina and František are drinking wine."
Translation:Kateřina a František pijou víno.
Different grammatical numbers. Also, the first phrase in your comment is incorrect, because it mixes a first person plural subject with a second person plural verb. The sentence in this exercise correctly uses a third person plural subject and a third person plural verb. Please read the Tips and Notes for each skill, when they are available (web only, but for both desktop and mobile; click the light bulb icon after you select the skill).