You cannot create a Nintex Form on Forms library.  Instead, create a SharePoint list and craete a Nintex Form. Then you can create variables, rules, also use can have many runtime functions. 

Thanks for responding , i have already started working on list as a workaround but here again i see there are very limited option available in responsive designer for rules , and same goes for classic mode as well but i have read that classic mode supports javascript hence custom rules can be added there , but classic mode looks lot of work  because i am dealing with around 100 columns ,so it will involve lot of efforts to one by one add custom javascripts for each column and its layout issue is also a big challenge to deal with.
its very difficult to achieve all the similar functionalities from old Infopath 2010 in Nintex form .
I hope its get better in future 

Hi Ullas,

You are correct, new responsive mode does not support javascript, but the rules are powerful and easy to use compare to classic form rule. All the best with your requirement.