Hello and welcome to Apple Support Communities Sarahwilso,
I see that you're getting a message that your card is invalid when trying to add it to Apple Pay. I understand your concern, and I'd be happy to help.
You've taken some good steps so far to resolve this issue. Now, let's have you take a look at the following article for some additional steps that can help. You'll want to also make sure that you bank participates in Apple Pay. You can check that within the same article as well.
Set up Apple Pay
Also, if you need assistance with changing your country in the iTunes & App Store the article below can help.
Change your Apple ID country or region
Thank you for using Apple Support Communities.