Just for clarification, which forms designer (Classic or Responsive) are you importing from?  Also, which forms designer (Classic or Responsive) are you importing the XML file into?

Hello, i am importing classic form.I tried importing into both classic and responsive but both the options do not work

You are only able to export and import a Classic form into a Classic form (and New Responsive to New Responsive. You can not export and import a Classic form into the New Responsive designer (and vice versa).

I’m assuming you’re going from a Nintex 2016 to Nintex 2016 environment? Are you exporting and importing from a list to list or library to library?  Have you tried exporting and importing into a test empty list/library?

Hi, I am trying to import from classic to classic .From 2016 version to newer version but in classic view. It’s a plain .xml file which i have originally exported from 2016 version.

Hope this answers your question

So exporting from 2016 and importing into 2019?