If you do not let us know in advance that you would like to cancel your reservation, we would have to ask you to pay the fee in its entirety.
"If you do not let us know in advance"=「もしあなたが事前に私達に知らせなかったら」
"that you would like to cancel your reservation"=「あなたがあなたの予約をキャンセルしたいということを」
"we would have to ask you to pay the fee in its entirety"=「あたなに全額を払うようにお願いしなければいけないことになります」
(日本語に訳すと多少不自然な部分がありますが、毅然とした態度は伝わる英語になっています。)