The order was canceled twice, and no explanation was given

It depends on the situation.  If the card failed validation, it would be the reason;  your card provider should be able to investigate that.

If you try to use an out-of-country card to purchase (i.e., an international card for purchase in the US), that would be another cause.  You cannot place an order in one country using a card not registered to a user in that country, nor can a delivery take place in a country other than that where the card is registered.

I think you'll find that Dell doesn't have a sales presence in Ukraine other than through authorized dealer/resellers.

I don't think Dell ever allowed the purchase and export of products to another country, but even if it once did, the ever changing (tightening) of export of technology laws for US-based companies would be the explanation.

If you're buying from Dell USA, note that the purchase agreement spells out the "not for export" restriction.

See #11 here

https://www.dell.com/learn/us/en/04/terms-of-sale-consumer-terms-of-sale

Hello. Thank you for the reply

You got it right. It will be case number two for me But I'm not sure why you can't do that. I am constantly buying things in the US and delivering them to myself on my own. First time from Dell, though.

Or, for example, I have a start-up that sells subscription-based services to clients around the world. The start-up is located in my country (Ukraine), and we have clients from more than 70 countries worldwide. Obviously non of our clients has a Ukrainian bank card. But there are no restrictions for them to buy things from us. 

You are correct. But there is no purchase to another country. The delivery is made to my name and it is made within the US. The only thing is that the card (which is also issued to me) is from a Ukrainian bank. Which, as I mentioned earlier, is a common thing in ecommerce. I should be able to use any bank I have, not only country-specific banks. Same as in every other store in the US.

Otherways, what's the point of having the same bank system for the whole world if for every purchase you need to open a bank account in another country? We already have a system like that, it's called local cash currency

Also, why when I put it into the cart now, it says:
"The number of years you have selected for Accidental Damage Protection needs to match the number of years on your Support Plan option. Please click the highlighted words to choose matching years of Support. "

And does not allow me to proceed further unless I pick an additional 5 years of support, which I don't need and which costs almost 15% of the laptop price?

Hmm. I see. Thank you. And if I pay with a local card, everything should work?