User Cannot Submit Form

I was just on the phone with a user who is trying to register for one of our events.

I walked through the process with her and it let her advance to each page, but on the final payment page if she hit submit it would tell her there are errors in the form and not let her submit it. The payment is optional, and it didn't matter if she selected an product or no product, it would not let her submit the form.

Any ideas why or how to let her save/submit what she does have?

I have just tested your form and did not find any issues to submit it, I did not select a product and was redirected to the thank you page as expected, here is a screen-cast about my test: 

In order to further investigate the issue we will need to replicate it or know what may be causing it, the notice about required fields empty is displayed as well when a field with a mask like a phone number or email, for example, are not properly entered, I would recommend you to kindly review carefully the data typed in the fields in order to view if anything needs to be corrected there. 

A field with an error will be highlighted on red, if you do not see a field like this, but still cannot submit the form, please kindly provide us some screenshots about it, this guide will help you uploading the images to this forum thread: How to Post Screenshots to Our Support Forum 

Attached images via email replies will not reach this thread. 

We will wait for your response.