Are you using the latest version of Form Plus? I tried this on Concrete 8.5.9 and did not notice any issue while installing it.

We're using v.2.3.6

Please use the latest version:https://marketplace.concretecms.com/marketplace/addons/form-plus1/ch...

That issue was resolved in v 2.4.1.

Hey there,
It worked, and we've installed the plugin successfully
However, we're getting the enclosed error when completing the form
Our contact form is herehttps://www.shikokutours.com/contact-us...
Thanks,
Max

fyi, the issue only occur when we tick the newsletter subscription box

Thanks for your email. Unfortunately, I was unable to replicate the issue you mentioned. I tested a form with an opt-in checkbox. When I tick the checkbox and submit the form, I didn't get any error. I was able to successfully subscribe to the Mailchimp list.

I recommend you double-check all your merge tag mappings.