Hi there,
Sorry about the delay in writing back to you. I just tested on the subscription form on your site and wasn’t able to replicate the issue. There’s no errors referring to the form on the site or in the browser. I have tested this three browsers (Chrome, Safari, Firefox).
Could you please send me over a screenshot of the error shown when you attempt to use the form?
The warning does not yet show up, but will do so in October 2017.
Here’s a screenshot of the message I received from Google about this.