Translation:I have a date this Saturday.
A date? In English, "a date" only applies when meeting someone with the possibility of a prospective amorous relationship. For instance: "I set up a date for my sister with my friend Alan ." Is that what "约会" mean? That's news to me . . . A date of this kind would be: "约会对象." The one here is "an appointment."
"约会对象" means the person whom one meets on a (romantic) date, which is also referred to as one's "date" in English.
And yes: it may be news to you (probably because you aren't a native speaker of Chinese), but "约会" is almost always used to mean "(romantic) date", and not to mean "appointment". If you use "约会" to mean anything other than a romantic date, then native speakers will look at you funny.
"约会" is strictly "romantic date" in common usage. Using "约会" to refer to something like a doctor's appointment, for example, would sound just as strange to most native speakers of chinese as the sentence "I have doctor's date" does to native english speakers: it sounds like what is meant is that you have a (romantic) date with a doctor.
It is funny how in the earlier parts of this course it does not let you say 'this Saturday' when comes to a future event. You get marked often wrong because they want you to say "next Saturday" (or whatever future day they write).
But now in this part of the Course it is always 'this Saturday' or whatever day.