Dear Cheryl Lee,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and I would like to let you know that I have tested the behavior in the PowerPoint for the web, where I am able to insert the YouTube videos without any issue:

In this situation, I would like to suggest you check whether you are having the same issue when you try to Insert the Online Video into the PowerPoint presentation via a different browser or InPrivate/Incognito window of the browser?
If the same problem happens, can you please check whether other users in your organization are having the same issue?
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia