Q: Why am I getting invalid payment method?

Beta software can't be discussed on these forums : https://beta.apple.com/sp/betaprogram/faq

But prepaid cards are unlikely to work, you need to use a proper card that's registered to the same name and address as on your iTunes account

you are currently in the US  you can't use the US store if you aren't if you are and you don't have a valid payment method try contacting iTunes Support and see if they can/will remove the payment details requirement : https://support.apple.com/contact

Then contact iTunes

http://www.apple.com/support/itunes/contact/

What's confusing ? Are you in the US ? You have to be in a country to use its store, otherwise you will be violating the store's terms (terms that you agreed to abide by). If you are, and if you don't have a valid payment method, try contacting iTunes Support

Factory resetting will make no difference, if you want to use a different account just log into that account instead - anything downloaded by your existing account will remain tied to that account

By 'here' you mean the US ? You could wait a few hours until you can talk to them on the phone, or you can also contact them via this email form : https://www.apple.com/emea/support/itunes/contact.html