Train in Spain

You must be looking at Renfe’s website. This is the reason why so many buy their tix from one of Renfe’s contracted ticket resellers at www.TheTrainline.com or www.Omio.com. They often add a small commission to the sale, but being able to buy your tickets without wasting a lot of time is well worth it.

Is the list in alphabetical order by country name? Have they used the Spanish form, "Estados Unidos"?

I can not find the +1 phone code for the US

Both Canada and the U.S. have the +1 country code. I encountered a website once that listed Canada but not the U.S. I guessed the software developers assumed that the codes were unique to a country and just picked the first country. If it is only the phone # field, you could try selecting Canada.

Thanks for the suggestion of using another train ticket service. Today I learned about iryo.com. I'll give the other sites a try since Renfe is frustrating. Why they would have a list of all countries with corresponding country codes on some drop-down menus and just have a drop-down menu with a long list of numbers in no particular order (excluding +1) is puzzling, especially on the page where you enter the payment method.

I don’t recall the problem. I bought multiple tickets within the past months. My payments were all accepted with a US Mastercard. I may have entered the country code directly on the phone line. Tickets are all fine, received in email and sent to Apple wallet. I have received email reminders and updates on at least one train time. No texts, but Deutsche Bahn never reached me either.

... weird, I've never inputted my phone number to purchase a train ticket in Spain. Why would it be needed for?

Some transportation providers send texts about delays, etc. I haven't been in Spain recently and don't know whether Renfe does that.

I just bought tickets from Cordoba to Sevilla using iryo.com which was recommended by my Airbnb host without any trouble.

Happy to hear you found a workaround.
Have fun in Spain!