It's hard to get a reservation at that new restaurant downtown.
It's hard to get an appointment at my favourite hair salon because they are always so busy.
予約が必要な場所がレストランなら、次のように言えます。
ーIt's hard to get a reservation at that new restaurant downtown.
「ダウンタウンの新しいレストランで予約が取りにくい」
It's hard to get a reservation at ... で「…で予約が取りにくい」
もし予約が必要な場所が美容院なら、次のように言えます。
ーIt's hard to get an appointment at my favourite hair salon because they are always so busy.
「いつも混んでいて、お気に入りの美容院で予約を取りにくい」
It's hard to get an appointment at ... で「…で予約が取りにくい」
ご参考まで!