。
Please make reservations one week in advance.
Literal
Reservation [topic-は] one-week before [at-に] please.
Note the topic-fronting: 予約は gets placed up front as 'as for reservations...' and the rest is the comment. お願いします closes the sentence as a polite request. 一週間前に is a standard time adverbial — the 前 here means 'before, prior' and attaches directly to durations.