So you are looking at:
passenger 1: ZNNQ7NFZ
passenger 2: MNXA0UFY
This is the cheapest biz itinerary on DL on ita for your given dates + flight numbers:
https://i.imgur.com/jrRaluT.png
Different classes than yours and different basefares, but that doesn't matter since I already see the mistake you made.
You have a different fare construction for pax 1 and 2, meaning 2 different basefares (=and 2 different fare classes) since ITA says there is only 1 seat left in the cheaper class for 1431$ total and one for 1474$
There is not a single OTA, airline website or any automatic booking engine that is out there that is able to book 2 or more passengers on the same booking with different classes. All passengers need to be booked in the same class. This is why you are always getting the error on the DL website when coming from ITA.
Workaround:
a) Book each passenger individually
b) Book both passengers together but in the same booking class by forcing the basefare / fare class to the more expensive fare for both of you, for example:
SEA :
L2580 ATL DL727 / airlines DL; f DL..KNXJ0NFY; minconnect 683; maxconnect 685
MBJ :
L360 ATL DL2725 / airlines DL; f DL..MNWA0UFY; minconnect 285; maxconnect 287
Put the 1st line in the "Departing from" line
2nd line in "Destination"
then it will force both passengers in the more expensive basefare where still 2 seats are left.
Thats what it would look like for the search you did in Z class (your search for passenger 1), but this one is sold out already.
SEA :
L2580 ATL DL727 /
airlines DL; f DL..ZNNQ7NFZ; minconnect 683; maxconnect 685
MBJ :
L360 ATL DL2725 /
airlines DL; f DL..ZNNQ7NFZ; minconnect 285; maxconnect 287