Cant open Nintex form in SharePoint list it going on blank page of SharePoint

Hi All,

I can't open Nintex form in one of the SharePoint list.

when I  chick "new item" in a list it coming up as a blank webpage. 

this should be coming up as Nintex form page. 

does anyone know how do I make default Nintex form when a user clicks a "new item" in a list?

Many Thanks