I am having the same issue as reported in the thread here,  

Same screenshot as was posted in that thread.

This is a newly built SP2016 farm using a Nintex Development license. Using IE 11, I created a new subsite, created a custom list, activated the Nintex Forms features at the Web App level (in central admin) and at the site collection level. I clicked the Nintex Form button on the new list and attempted to publish but got this error. However, the form was actually published despite the error. So it is more an annoyance than a real issue, but do you have any ideas how to stop the error from happening? Users don't like hearing "I know it looks like an error, but really it's fine, don't worry about it" if you know what I mean...