Hi,
Is this present with any list or only this specific list? And is it present in the current site or any other site also?
If it is with current list only, try deleting the list and create a new list and then create Nintex form.
If you see the problem within the current site only, check if there are any errors in "Nintex Forms" app. If everything looks good, give a try by uninstalling the Nintex Forms app install again. Make sure you delete the "Nintex Forms" app from first and secondary recylebins, wait for some and then install again.
SOLUTION FOUND
It would appear that Nintex Forms 365 does not like the new version of Lists from MS
I have deleted the 'new lists' and added 'old lists' via the 'Custom List' option in SharePoint 365