what would you say, is this supported: First I create a form, classic or responsive, add columns, panels and tab-like button navigation to show/hide panels. Then, I would like to edit the NewForm to add a webpart, like using the standard SharePoint ribbon action.
Is this supported? Reason I ask is that after adding form panels, editing the form with SharePoint Ribbon action generates an error message. Nintex Forms editor works fine.
If you are working onPrem this is possible.
If you are working in SharePoint online it might be impossible because your form should be displayed in the App Web of your Nintex Forms application. This App Web may not give you the possibility to add additional web parts to the page where your form is displayed. (As i have no access to SPOnline at the moment this is only a guess)
However could you share the error message?