Thank you for reaching out to the Community.
I have taken a look at the RSVP button on that email and see exactly what you mean. It appears that you did not assign a URL to that button. That said, you can make a copy of that email, correct the link and resend it to your list. Click Here for instructions on adding the correct URL to that button.
I sure do hope that this helps but please let us know if you have additional questions. Thank you for using Constant Contact, have a great day!