Web Part pages are a type of pages in SharePoint that contain web part zones. Most of them do not have any rich text area. These pages look like this:
The page does not have an area to type text; you can only add other web parts, app parts, and apps from the store. For ShortPoint to work, it needs rich text area.
Click inside the content editor area to activate it. Then open Insert Tab from the ribbon.
Do you see ShortPoint Insert Button shining? Yes, it is there waiting for you now :)