That's used as one word, right? It's to "book" a "cancellation" for future. The contract is not cancelled immediately, but is to be cancelled on the certain date the customer assigned. When used in the context of finance referring to CD, or time deposit, which is usually renewed automatically, 解約予約 means to stop the renewal on the next maturity day. CD, time deposit:定期預金（ていきよきん）

Unlike 解約 or cancellation, no refunds are issued for your purchase. However, your rights, account or membership will remain active until the last day of your billing cycle. Therefore 解約予約 will be "Stopping Automatic Renewal (of something)". In some cases you'll have to continue to pay until certain date, again, your rights will remain active during your billing cycle.

解約　means "cancellation". 予約 means "reservation". ex: I cancelled the resevation.私は､その予約を解約しました。

= cancellation