I recently bought the plugin Paypal Recurring Payments. I was testing the demo page and when you click on “cancel subscription”, this error appears :
Hello, @jellowfishx
Thank you for using our plugin.
Please allow me to note that currently, our developer team is working on the Demo Page for this plugin.
It may provoke some mistakes, I’m afraid. However, please note that this does not affect the operation of the plugin in any way.
I will let you know about fixing this error immediately after finishing the works by our team.
Thanks for the understanding 
Best regards,