Hi again @allan1995 and sorry for the delay.
By any chance, when you download a file to your macOS, can your machine open it?
There is no built-in pptx preview into S3Drive, it relies on a software installed on your macOS.
If you use: "Open externally", S3Drive tries to find file association between pptx and default software and then either opens a relevant software or displays: "No installed application can open these files".
Can you please confirm whether downloading a file first and then opening on your macOS works? What software you have installed to preview pptx?
Thanks!