I think you need to turn it off in OneDrive. If you can see your OneDrive folders in File Explorer, right click on the folder and choose "show more options" and then "properties" (some versions of windows will just have properties). On the General tab, unchecked the box labeled "Read-only". You may get a pop-up about applying to subfolders - you should probably choose yes. Then click. Ok. Restart PowerPoint and open the doc again.