There is no error message that comes about. After my sister keys in the details to confirm her residence it simply goes back to the previous page and the request does not get processed. I'm not really sure how else to describe this, is this a glitch with spotify?
Have your sister open the email in an incognito browsing session. She'll need to log in with the account associated with the email address the email has been sent to. Keep in mind that everyone on the plan must: