You can buy it online as two separate tickets. KL still should be able to check your bags through, if you have any.

If you call or chat with reservations and have them book it as a broken fare mixed-cabin, additive pricing, on a single PNR/ticket, or book as two separate ones, in reality doesn't make much of a difference...oh, other than cancel fees, as this is outside of US origin for both, so if you cancel it you'll be hit with double redeposit fees. Not sure if the AMS "origin" vs connection will price differently for the taxes/fees also. So, there may be difference in fees, and will be a 2x higher redeposit fee if you have to cancel (or change) it later, but miles-wise it'll be the same cost booked as one ticket (painstakingly slow and difficult) via reservations, or easily so booked online as two separate tix.

Book it yes, easily no. IME such additive-pricing itineraries, more often than not, are anything but a breeze to book. Probably depends on how competent and well trained (or not, as is more often the case, now more so than ever) agent you get. Esp. if here it's MS "logic" that is preventing this, I don't think the override to that, and doing it as additive pricing, is something that agents are trained in, so expect a lot of hold time while the front-line agent is on hold internally to GTS.

If DL doesn't show it as available in X, then it's not available in X. At least at this time. So, you can either book it in O (additive pricing of the two flights), one ticket via reservations (may take several hours, given how hard it is to get through to DL nowadays), or two tickets online. Seems like here DL is forcing you to pay additive pricing. Or you can just book the long-haul flight on dullta.com, and book the short "domestic" EU connector using EUR, on KL on another airline (like AY presumably), whichever is the lowest cost and best schedule that works for you.

You may be able to book it as a through-fare both flights if this is not a case of married segment logic, but that may take 10s upon 10s of hours over many days and weeks of trial and failure until you get through to someone at DL who will actually do so for you. Probably not worth the time and hassle.. And there is a good chance this is a case of MS logic here, as you said that other dates this is available as a through-fare. Remember when booking awards with DL you need to have good flexibility, good to have at least +/- 3 months in your travel dates, and when booking non-DL flights esp. it's usually best to book it straight at 331 days out. So plan 11 months ahead and be very flexible with your dates, for the best chance to use your DL miles at a fair value.