Often the reason that a video cannot be played on TED.com is because it came from a "TEDx" event. You can learn more about TEDx here:
Occasionally, some exemplary TEDx talks are chosen by our content team to be featured alongside talks from official TED conferences. Here is an example of a featured TEDx talk:
When a TEDx talk is chosen to be featured, that elevates it to "official TED talk" status. The speaker is given an official bio by our content team, and the talk is added as a file on TED.com (as opposed to an embed). This makes it downloadable and viewable everywhere.
At the same time, we're also embedding TEDx content post-2018 from YouTube onto TED.com, so that it can be part of a larger, holistic TED library. However, embedding a TEDx talk is not the same as being featured content, so "official TED talk" status is not granted to it.
This non-featured content is only embedded on TED.com from YouTube, not added as unique files. This means that the content isn't downloadable, it won't have subtitles, and any restrictions upon YouTube content will also apply. In other words, if a school network blocks YouTube, or a local ISP does not allow access to YouTube, the content will not be viewable even though it is shown on TED.com. This non-featured TEDx and TED-Ed content is also currently not available on the TED Android or iOS app.