There may be a few reasons for this issue.
1. Check your bandwidth
Check to make sure your internet connection is working and can handle streaming videos. Most Khan Academy videos require less bandwidth than live-action content, but we still suggest 1.5 Mbps bandwidth per device playing standard-definition video.
2. Check your browser is up-to-date
If neither of those are issues, check that you are using a web browser supported by Khan Academy. Whichever browser you choose, you may want to upgrade to the most recent version. A technical director at one school district who had reported problems with watching videos using Internet Explorer has informed us that Installing or Upgrading to IE 11 resolved his district's problems.
3. International Problems
Finally, YouTube is currently blocked in China and Iran. However, you should be able to watch our videos from China as Khan Academy uses a different video player to show the instructional videos.
You may be able to access the "KA Lite" site from Iran instead of our full site https://learningequality.org/ka-lite/.
If none of the above is your problem, please submit a helpdesk ticket by following the instructions on this page