Keynote Error Dialogue When Opening .pptx I opened keynote 7.3.1 on my Mac (10.12.6) and navigated to the Desktop to open a .pptx document.
This resulted in an error dialogue stating that it could not recognize that file format (.pptx).
What would cause this error and how could it be resolved?
Try to locate the .pptx file in Finder (instead of the Desktop), right click on it, then move the cursor onto Open with… and select Keynote to open the Powerpoint.

If that doesn't work let us know.