Delta does not offer bassinets on domestic flights and I'm fairly sure the 321 does not have a bassinet position but it's annoyingly hard to find this information.

You are certainly welcome to book a third ticket in the name of the baby and use a car seat in that seat. Be aware that children under 2 can't sit in a seat with an airbag, which includes rows 1 and 10 on the 321neo.