Hello there,
I'm the developer of IFN, sorry about the PHP warning, it's fixed now.
About the Paypal error, it's due to the fact we only accept what's called InstantPayments, wish requires one of the following :
1. Payment is funded from your Paypal cash balance
2. Payment is funded from your linked Credit Card
3. Payment is funded from your linked debit bank account but ONLY if you also have a Credit Card on file with Paypal as a "backup funding source".
I would need you to specify if you used a debit card or not, if so using a debit card can't apply to be an instant payment. So that would be what causes your payment to fail.
It would be great if you could get back to me on this, so I can help you solve this problem and access our service.
Wishing you the very best, Xooor