"Κάνε κράτηση ένα τραπέζι για το απόγευμα!"
Μετάφραση:Reserve a table for the afternoon!
This is marked wrong because, in English, "reserve a table" is somewhat figurative. When someone says "reserve a table," they don't literally mean that you should pick a specific table—you wouldn't call a restaurant and ask to reserve the third table on the left for the afternoon. Instead, "reserve a table" means "reserve space so that you're guaranteed to have some table."
The phrasing "reserve one table" could make it sound like you're asking someone to reserve a specific table, so that's probably why it's marked wrong. But (a) this is just conjecture; (b) people would certainly understand you if you said "one table" (although it might sound weird); and (c) maybe it's just an error :)
By the way, the phrasing "make a reservation for this afternoon" would be fine. "Make a reservation" is commonly used (though "make a reservation for a table" sounds a bit weird because it's redudant).