This is a well known Delta issue.

It has nothing to do with HA.

Delta has changed HA a few months ago to long sell. But, it has neglected to notify the agents of it.

Yes, you can book it. But, you may need to call back a few times (for me it took over 4 hours on the phone with DL to book a 30 minute RT flight!!) until you get a less-incompetent agent.

Tell them that they need to do the long sell process. You may need to keep telling that to them a few times, if not more than just a few, to make them actually do it and get past their habit of telling you the well-practiced lie of "not available."

They will set it up, it will be waitlisted. Then check a day or two later and it should come back as confirmed, then you can book it.

If they don't enter it right, it may be waitlisted forever, and then you may need to get a more competent agent to fix it or re-do it (pray that the inventory is there still then).

But, expect quite a few calls on this, as even when the PNR is setup and waitlisted, it can't be ticketed until it comes back as confirmed. BUT, dullta.com now longer shows the waitlisted status online properly (waitlisted or confirmed), as it used to; that has been also taken away (DL is really good at frustrating its customers in its never-ending march backwards when it comes to IT and functionality, usability). So, it's a guessing game now of when it's ready to ticket. You just have to keep calling back twice per day, and within 48 hours or less it should come back as confirmed and be ticketable.... if the agent set it up right. And that is a big IF (I've had some wrongly set-up DL waitlist flights take 2 weeks and not be confirmed, even though the inventory for DL from the partner airline is there, because they were set-up wrong on the DL end but no one had a clue how to do it right; basically, if you get an NW agent (HIBRES), it should be done right, but if you get a DL agent, esp. the horrendous SLC that messes up 1 in 2 simple bookings, count it to be messed up).

Oh and also, enjoy how now all HA flights mileage is 150% of what it used to be. The "low" is still 5000 in the award chart, but, DL is now charging you a "mid" level of 7500 always. The low on HA only exists in the chart, but not in reality anymore. (Only a matter of time until so will be for DL itself as well.)