PowerPoint can't accept an Office file dragged onto it?
In macOS, what to do if you want to drap or copy a image into PowerPoint and it just says:
PowerPoint can't accept an Office file dragged onto it. To insert Office content here, use Insert > Slides From or use Insert > Object.
Well, it might just be that your picture file name is too long or has some special characters in it.
I say this because I recently faced this problem and I found that drag-n-drop works in Word but not PowerPoint, then I change the image file name to aaa.png
and it worked again in PowerPoint.
Thought I mentioned this and hope this helps whoever stumbled upon here.