I have setup a paynow subscription button at http://www.sfuwh.org/club/pay but several users have complained that they get the error "We are unable to modify your subscription. This may be because your existing subscription calls for no more payments to be made or because it has been cancelled or completed." But neither I or the user have any record of an existing subscription. I was able to subscribe using my personal account without a problem. I've tried search the web and this site for the message but didn't find anything useful. We aren't using trials and I can't find these people in the list of subscribers in my merchant account.
How do I resolve the error? Thank you.
With this error I would recommend filing a ticket at www.paypal.com/mts, they can look into what is exactly happening and get to the bottom of it for you.