If Save a Copy/Save As and Export do not work, try turning on AutoSave and saving to OneDrive location.  If still getting an error when attempting to save:

1. Turn off embedded fonts in the Powerpoint document under Preferences > Save > Turn off embedded fonts

2. Turn on Autosave with One Drive as the target save location

3. Open from One Drive autosave version and then you can export/Save As without issue