In other words, DL site is not updated. You may want to check other sites for availability first. Like checking AF to see if they show availability on DL's dates.
AS has the same issues on virtually ALL CX seats. It would allow you to book but error out at the payment page. The solution is to check QF and BA sites (AA's is useless as AA also shows phantom seats) - if both QF and BA show availability then the seats shown on AS is bookable. Else it is a waste of time. Though the most annoying thing is, QF and BA show seats but AS does not have it. In this case it is a 50/50 chance an AS agent could find it. Seems a competent old-hand, would do a "long sell" to see if CX comes back confirming the seat if the date QF and BA both show availability. The thing is, finding a competent agent who is willing to do a "long sell" is 100% a luck of draw.
Before Pandemic, the agents at the partner desk were very competent and willing to help. I guess many of them left the industry for good during those 3 hard years.