Likly a CSS issue with your Avada theme but CSS issues on mobile are always tough to chase down. What it looks like to me, is your site has some Javascript that only runs on mobile devices which is affecting the Submit button.
I have an app on my phone which allows me to view the HTML source of a page in Safari and the input element for the Submit button is there but it is non-functional. So there is something your site is doing for mobile users which doesn’t happen for desktop users. You’ll need to figure out what your theme doesn’t differently between the two.
The Avada theme you are using defines a CSS class for “error-message” which is also used by Google. That is why you are seeing the “Must be a valid URL” in red text when the page first loads. If you set up a CSS prefix when you define the form this won’t happen but any Custom CSS you’ve developed for the form will need to account for the prefix.
