I thought of flagging the absence of the article the as a mistake: We’re booked on the
But then, I found plenty of both ways: "I am booked on the flight 9999" and then also "I am booked on flight 9999".
I don´t know how the mind of the testmakers, so I don´t know if they would flag the article on that one. Another possibility, fatter chances here, is that
We´re denotes present time "we´re" is short for "we are" (that one would be a "catch" - casca de banana).
Because to Brazilian ears and eyes it would be similar to were (that is a past form), so the student would think of it (we´re) as past tense as well, and it´s not, it´s in the present.
We´re = we are
Uh Oh! If "we are" and "booked" is a past form, then there´s an impasse between the verb tenses (are - booked), perhaps that´s why "we´re" is not used often
in formal writing. https://www.macmillandictionary.com/the ... hort-forms
Of course, to me "we are booked on (the) flight 607" would be understood, it´s understandable. To me "booked" in "we are booked" could work as an adjective, meaning "the state of being booked".
But the testmaker(s) there might have thought different, and they can. On the ground of the reasoning I have explained above.
Anyway, it´s a hunch, a guesswork and I could be mistaken. So, let´s wait for more advanced learners, natives or English teachers have their say on that one.