Why "he drinks afternoon" wouldn't be right, please?
Because in English, we say "in the afternoon" to indicates when something happens, not simply "afternoon" or "the afternoon".
So when you're translating from Greek to English, you have to insert a preposition because English expresses time differently from Greek.
Lingot to you for including a screenshot!
Some people are wondering about why the Greek doesn't have the preposition "in." That was new to me, too. This situation happens with languages: It's similar to the Spanish verb interesar, "to be interested in," for which there is no preposition added, e.g., me interesa beber en la tarde, "I'm interested in drinking in the afternoon." There are probably many examples from many languages one could add. At issue is that English works differently, in these cases requiring the preposition "in" where Greek and Spanish do not.
It's not quite so straightforward. The preposition is needed with clock times and calendar dates, for example, you say Θα πάω στο κατάστημα στις 9:00. But other measures of time, like απογεύμα, δευτέρα, or χειμώνας, you don't need the σε. I think that trips a lot of learners up (including myself, especially a couple of years ago).