UK senior rail discount card

I've never tried to buy one on line despite working on the railways for years. You should be able to get one on demand at a staffed ticket office - you don't need it in hand to buy tickets, just when checked on trains and in stations.

If there are 2 of you have you also considered a Two Together Railcard? If you can live with the 9:30 M-F restriction you can save money over two Senior Railcards.

are you on the official website? https://www.senior-railcard.co.uk/

I seem to remember having this issue pop up within the last year. Perhaps it was in applying for the UK ETA. The answer in that case was what is called the machine readable zone on your passport. In the case of US passports that’s the two lines at the bottom of the picture page. It starts with P and has lots of chevrons “<<<<<“ and your name. I entered the first 30 and it was accepted.

For further info Google “what is my 30 digit passport number” the AI result was what I remember using.

I hope that works for you and enjoy your trip.

A small point, but are you eligible?

From their website:
https://www.senior-railcard.co.uk/about-the-railcard/are-you-eligible/

Aged 60 years old or over
Have a valid UK Driving Licence, Passport or EEA national Identity card
Can be bought online or at a station

I don't think you need to be a citizen of any particular country to buy a senior rail card, if that is what you are implying. I (US citizen) bought a senior rail card less than a year ago year, in person at Waterloo Station. I haven't seen a report of a policy change on this forum.

This is a link to one of the FAQs related to the senior rail card: https://www.senior-railcard.co.uk/help/faqs/the-system-says-my-passport-is-invalid-what-can-i-do/

Here's the key text [emphasis mine]:

"The system says my passport is “invalid”. What can I do?

"We accept machine readable passport codes from all countries except for those issued in the Philippines and Bangladesh prior to 2010. So, if you have a valid passport your passport code should work."

I continues with more details about passport numbers.

I also entered the first 30 digits on the bottom line and it worked.