Cannot create YouTube slide

I’m using the free version 3.28.0. I have an existing slide show, I try to add a slide with a YouTube link to a video I own, and it doesn’t work. I can add an image slide no problem, but YouTube and External URL do not work. Basically I click on them when adding a slide, insert my YouTube link, and when I click on “Add to Slider”, nothing seems to happen. I thought it might be the format of the link, so I tried it these two ways:


Clicking on “Add to Slider” does not do anything. Same thing with External URL….it says to click on “add to Slider”, but nothing happens when clicked.
Do I possibly have the YouTube format wrong?
Is there some other setting in WordPress I may have set wrong?
I created the slide show several years ago, and would hate to have to recreate it, and even if I did, it still appears I can’t add a YouTube video.

Thank you for any help!
Brad