USER id is missing

I tested the form and tried to submit

But it keeps saying user ID is missing

what does it mean

this app seems great but can't submit form

also if I want to incorporate payment..which plan should I use 

You are getting the error because the PayPal checkout widget is not connected to a PayPal account. To fix the issue,  select the payment widget, click on the wizard, then Connect with PayPal to authenticate your Account. 

Try this and let us know if the issue persists.