I believe that for 2013 it still might be limited to using Internet Explorer as the browser. Have you tried running it with that?

For future reference, here’s a link to Nintex documentation for Web Browser Support:

Web browser support (nintex.com)

Additionally I think that certain browsers were supported after certain versions for Nintex, so you may also want to check and see which version you’re running and what the latest version is. If there is a several year long gap, an update might allow you to use the new browsers. 

Hi , Thank you all for your time in replying. As per the documentation, it supports IE 11, Edge & Chrome, latest version in Windows. I have updated my browser to the current latest version as well. Still I am unable to get rid of the error message, which prevents me from editing my forms. Please help. 

1. I did you actually try to use IE to open the Form Editor? I’m not interested in what the Documentation says, I’m interested in the results of you taking direct actions.
    
2. I didn’t suggest updating your browser. Which version of ***Nintex*** are you running? Is it Updated?
    
3. If the problem persists, if you have done all of the above, contact Nintex directly through your customer portal and open a ticket.

Below is’ a link to the Nintex release notes information, which documents Nintex product updates/fixes/changes.

Nintex Forms for SharePoint 2013 release notes

Hi friends. How are you all. I hope you all will be fine.It is a nice post. I like all the comments.