Hello,
Thank you for contacting Rank Math and bringing your concern to our attention. I’m sorry for any inconvenience this issue may have caused you.
According to Google, this could be caused by rendering issues in Googlebot, so you might not be able to fix it, but you should ensure that the video is reasonably prominent on your page.
For troubleshooting, please use the URL Inspection tool and click Test live URL > View tested page > Screenshot. If the video is outside of the initial screen position, you’ll need to try another test method.
Here’s a link for more information on this issue:
https://rankmath.com/kb/fix-google-could-not-determine-the-prominent-video/
Hope that helps.
Thank you.