<frame> or <iframe> elements do not have a title
Give your iframes a <title>
If you use the Webflow Youtube element there is no title element added and it is not possible to add one. Vimeo supplies a title, so you get your Lighthouse points, but it's <title="Vimeo embed>, which isn't very helpful.
You can avoid this error by using the generic Webflow embed element. Copy the Embed code from Youtube and put it in the embed element in Webflow. Then and add title="your_video_title" .
Google Lighthouse says
Users of screen readers and other assistive technologies rely on frame titles to describe the contents of frames. Navigating through frames and inline frames can quickly become difficult and confusing for assistive technology users if the frames are not marked with a title attribute.