This documentation page is for Quform version 1 and may not be applicable for Quform 2 click here to visit the documentation for Quform 2.
If the popup does not work or when you click the link or it takes you to another page, then you may have a JavaScript error on the page, please check and resolve any errors.
In the Quform global settings at Quform → Settings on the WordPress menu, check that “Disable Fancybox output” is unticked and check that “Enable Fancybox” is ticked.
Another possibility is that you already have Fancybox loaded in your theme for another reason, if this is the case it could be conflicting with the Quform Fancybox script. You could try ticking the box “Disable Fancybox output” in the Quform global settings at Quform → Settings on the WordPress menu.