The event we were planning to hold at your venue has been postponed, so we are going to have to cancel our reservation.
この場合、次のような言い方ができますよ。
ーThe event we were planning to hold at your venue has been postponed, so we are going to have to cancel our reservation.
「あなたの会場で開催する予定だったイベントが延期になったので、予約のキャンセルをしなければいけません」
to hold at ...「…で開催する」
venue「会場」
to be postponed「延期になる」
ご参考まで!