Render operator is located in top row of app Render > Render Animation (Ctrl+F12).

[![enter image description here][1]][1]

Check if under Output Properties > Post Processing > Sequencer is enabled.

[![enter image description here][2]][2]

If it is not the case attach simplified blend only with a short sample of movie, so we can check. But if you are able to render with different codec I'm not able to help since issue lies somewere in your PC condition.


  [1]: https://i.sstatic.net/11QM4.png
  [2]: https://i.sstatic.net/xNZtG.png