Q. I can't seem to make a voiceover PowerPoint video on my Chromebook.

I haven't tried using PowerPoint directly for this on a Chromebook, but there is a Screencast-o-matic app, also known as a Chrome extension, that you can install following these tutorials to complete the assignment.
Installing the app - https://youtu.be/HBElJ_79Teg
Using Screencast-o-matic to record a PowerPoint - https://use.vg/Om40Co