If you hold one of the Delta cards that offer a companion certificate you need to book using a combination of eCredits and/or payment with an Amex card. Playing these games is the only way to apply the Capital One travel credit toward those flights.

You could hold out until after the companion certs are used and then use it on the next cash-only booking, but between certs and award ticket redemptions it might push using the $300 Capital One credit uncomfortably close to the expiration date.