Why can't Quicktime play, or convert and play, a PowerPoint that has been converted to a Quicktime movie?

I used Keynote to open a PowerPoint presentation (.ppsx) that has embedded videos. The videos play about 4-5 seconds, and then freeze. I try exporting the PowerPoint, and note that exporting it to QuickTime is an option. I watch the presentation (which I've seen on Windows) run by in the export window, including all the video. It generates a good-sized file (.mov). I then double-click it to open it, and QuickTime starts opening and converting it, but then ends by saying I need more software, with an option to jump to the Apple Web site. I land in a page about QuickTime, but with no details for such a problem as I have.

I can play the QuickTime in VLC, okay. Just not QuickTime.

It sounds as if the movies embedded in the PowerPoint presentation are not playback compatible with QT X AND your system is not configured with the Audio/Video codecs required for the "convert" OS embed to transcode the content using third-party codecs. I.e., this is why QT indicates you are missing "software" and sent you to the web page that refers to a limited number of "sponsored" codecs that may or may not include the one(s) you need.

Basically you have two options. You (or someone else) can analyze the MOV file to discover the specific codec(s) you are missing and then install them (if available) for normal MacOS QT conversion or you can use a dedicated third-party app like HandBrake (supported by the same open-source codec package that allows VLC to play your file) to convert the MOV file directly to QT X compatible media content.