Hi there @ARSC!
Don here from Shopify.
Could you provide us with more info here so we can try to figure out what's happening here?
If you could share a link to your store, and let us know what the error you are seeing when adding a payment method and selecting a plan, that would be great.
It would also be helpful if you could share a screenshot of the error if possible, as well as letting us know which payment method you were attempting to use.
The more info we have on the situation the better we'll be equipped to provide support here.