That page isn't rendering properly for me on any of my browsers, but I did see one that listed 16GB of RAM and the direct link to it is here: https://www.dell.com/se/foretag/p/latitude-15-5590-laptop/pd?oc=s042l559015ndc&model_id=latitude-15-...
Also, look for systems that are marked as Customizable rather than "Ready to ship". The latter are pre-defined configurations so that customers can get them quickly, but if you find one that is listed as Customizable, then even if it lists 8GB on the main page that you found, you might find that you can change specs like memory, hard drive, etc on the customization page. If you can't get the exact configuration you want yet, it's possible that Dell will offer more configuration flexibility and/or more pre-defined configurations soon. It's not unusual to have fewer choices right at the beginning of a product launch. Good luck!
I think I might have visited that page before but it does not load properly for me and it doesn't seem I can use it as intended. When I click "Customize and buy now" I come to another page where it just says "Service. Method not allowed.". Nothing more.
Hmm found it. Apparently is was some settings in both my browsers (FF and Chrome) that prevented the page from displaying properly.
Why is the hard drive size not customizeable do you think? I think I would prefer 512 GB.
Again I'm looking at the US version here, but different initial configurations allow different components to be customized. Some models offer configurations that are called "Fully customizable" where you can change everything, but I don't think that's available on Latitude systems. But on the US site I'm able to select a 512GB NVMe SSD. You could try contacting Dell Sales in your region via phone or online chat; they may be able to get you the exact configuration you want.