Trouble paying for flight online

This is abnormal. What about asking your credit card company?
Another option is to try to buy it on Expedia as long as it is the same price.

I think that you could call them and make your reservations. Most airlines will waive the fee if the individual has tried to make the reservation online and been unsuccessful. Even if they don't waive the fee, you probably don't need this additional trouble.....

Here are a few things I would check first.
1) Are you on the USA KLM website?
2) Does the credit card you are using generally work for foreign transactions without difficulty?

I've used the USA and Canadian websites before without difficulties so I am a little bit of a loss as to what is happening.

Good luck to you!!

Just did a dummy booking and I got to the payment page. Is it not taking your credit card?

Flights to/from where on what days?

Do you have a pop-up blocker and/or adblocker installed in your web browsers? Could be your browsers are both blocking what it thinks are ads. If using Google Chrome, try an Incognito Window (Ctrl Shift n) and try again - ad blockers normally don't run in incognito unless you explicitly enable them.

Are you sure you can't book on Delta's website? They are KLM partners.

Did you telephone the KLM US customer support and talk to a real person (wherever they are)?
https://www.klm.com/travel/us_en/customer_support/customer_support/contact/about/flight_bookings.htm

Call the KLM Service Centre: 1-800-618-0104

As others note, Delta is KLM's joint venture partner along with Air France and code share flights:
https://www.delta.com/content/www/en_US/traveling-with-us/where-we-fly/flight-partners.html

Thanks for all the suggestions. I am on the US KLM site, and the problem is not my credit card--it's that it won't take me to the payment page where I can enter the card. Before I call, I'm going to try one more thing and try to book a simple dummy trip, e.g., straight round-trip to Paris. (I've been trying to book an open-jaw: Portland to Paris, Ljubljana to Portland.) If I can get to the payment page then, that tells me it's probably an issue with the particular flights. And I'll be back to calling to make the reservations, but at least I'll know a bit more first.

And indeed, I can get to the payment page no problem when booking a simple round trip. I will report back when I find out what the issue is.

Out of curiosity, have you considered booking this open jaw with Condor instead of KLM? You can return (some days in May) from Ljubljana to Frankfurt and Frankfurt direct to PDX. Not sure about the fares though. You should be able to find an inbound to Paris with only one stop. Try the Google Flights website.

I tried to make a dummy booking, open jaw, for your route and it took me to the payment page.

What is the actual error message you're getting? Have you had any trouble with making other online purchases? Are you using the multi city option?

If you couldn't book any trip at all, I'd ask if you tried doing this on a different browser. I know that there are at least a handful of major, commercial websites which for whatever reason will not allow me to process a transaction on their site via one browser. I always have to switch to another browser.

Since you can make it to the payment page with the simpler trip reservation attempt, I am stumped.

My only additional suggestion is to clear all cookies and log out of accounts that follow you around the web on different browsers or computers. Like your google account. I’d use incognito mode on the browser. Sometimes pages seem to get “stuck”.

If it didn’t involve credit cards I’d say go to a public computer like the library and test it without logging in to any other account.

You could try booking it on the Delta app on your phone. I haven't used the Air France or KLM apps, but these should also work.

Solved. Sort of. Called KLM customer service, by now hesitant to book without knowing what was going on. They couldn't tell me, but sent me to web support; after 45 minutes of hold music I gave up. Meanwhile, I tried booking on Delta, since these are largely Delta flights. I only tried to go through KLM and Air France because they would credit miles to my Alaska card, but miles are not that important to me. At Delta, I got all the way through everything, including seat selection, entered my credit card, and THEN I got the error. (By the way, the error messages are all generic, e.g., KLM: Unfortunately, an error has occurred--please try again or contact us.) Delta customer service had the nice we-will-call-you-back option, and after an hour I got to speak with a pleasant lady who informed me that my reservation had gone through (yes, please, I did want an email confirmation!), but the problem is that they cannot confirm the flight from Ljubljana to Amsterdam on Transavia. So... I have confirmed flights from Portland to De Gaulle via Seattle, and from Amsterdam to Portland via Seattle, but no confirmed flight from Ljubljana to Amsterdam. Delta claims they will notify me if/when it is confirmed or changed, but also told me to check back periodically. OR I could void the whole ticket and start over. For now I'm holding, but no idea if that's the best thing to do.

Michele -your resolution has me a bit confused, namely that Delta can't confirm your Ljubljana to Amsterdam flight on Transavia. I think there may be a really good reason for that, namely that I am almost certain that your flight is on Adria Air. We were actually going to be on that until we switched so we flew to Zurich for our United flight back. Try https://www.adria.si/en/ and see if you can find yourself in their system. I did not even see any flights from Ljubljana on Transavia.

Adria airlines does not have any morning flights listed; Transavia does fly out of Ljubljana, and I see the 9:10 am flight I theoretically booked is listed, so I'm not sure why Delta can't confirm. I don't understand how they can list a connection and sell a ticket (took my money no problem!) but not be able to say I have that flight. Think I'll call back tomorrow when my head clears.

I had the same problem with Iceland Air a couple of years ago. I finally called and told a very nice customer service person my problem & how frustrated I was and she booked the tickets for me and waived the fee. Wouldn't hurt to try. Good luck!

FYI, Alaska Airlines Mileage Plan is discontinuing its association with both Air France and KLM, so any ticket you buy now for travel May 1 or later will not automatically earn miles. Big bummer, but it's already hard to redeem miles on these airlines without Delta in the mix, since Delta flies the trans-Atlantic leg.

Cancelled this itinerary (thank goodness still within the "risk-free cancellation" time) and am starting over. Thanks everyone for your help. I have booked flights online before, but I learned a lot through this one. To help others, here are my biggest lessons learned: 1) Next time I should call the airline right away when there is an issue, at least to find out the problem, before assuming a web site issue and pigheadedly trying to force a booking, because 2) apparently flights can be selected and even paid for (on Delta) that aren't able to be confirmed. And 3) when trying to cancel a booking within a free cancellation period, allow plenty of time before the deadline. Fortunately Delta allows more than a strict 24 hours--until midnight the day after booking--because the web site would not let me cancel online, and it was a 4-and-a-half hour wait for an agent yesterday. Also note that although Delta charged my card right away when I booked, I was told it would be up to 7 business days before I would see the credit.