When trying to publish the Form, this error might occurs:
Failed to Publish.

1. Navigate to Nintex Form hidden library. https://SiteURL/NintexFormXML/
2. Verify that all the files here has been checked in.
3. If there are files has been checked out, then it needs to be check in. 
4. Resolve the error during the check in (if any). 

Possible error during the check in:
Sorry, something went wrong
You must fill out all required properties before checking in this document. 

Those error above indicating there some required additional field that does not created by Nintex has been added to the Nintex Form hidden library. 
As resolution, go to the Library setting and set to No for the required fields.