A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
On the View tab, choose Slide Master.
That will take you to the layout the slide is based on. If you can select the header there, delete it.
If not, scroll to the top of the thumbnails on the left, click the topmost one and see if you can select and delete the header there.
If neither of those approaches works, make a copy of the presentation, delete all one or two slides that show this problem and save the file to the cloud; share the link with us so we can have a go at fixing it.