Opt-in Form Problem

I am trying to build a custom opt-in form on the sidebar. Upon doing this, I get weird results. WHEN the form is blank, and someone submits it (this is wrapped in a normal form part) it will go perfectly fine to the page where the form is.

Though, if you test it and try to enter any information into the opt-in fields, then it will try to go to the same page upon submit and WordPress will show a “page not found.”

Why is this happening? Can I not submit from page to page?

source: https://www.quickestpropertysale.com