Cannot add form fields

Hello. I have a previously created Ninja Form that works great. But I have been asked to add a new checkbox field. When I attempt to drag the checkbox template field I get the circle/slash icon on drag and cannot drop the item to create a new field. I have tried creating a new form as a test and the same response happens. I have also tried multiple browsers as well as deactivate/reactivate with no luck. the only modification to the site since the original creation of the form was modifying the site URL to have the site respond to the production URL, vs. the staging URL we developed on. Any thoughts?

https://wordpress.org/plugins/ninja-forms/