У вашего броузера проблема в совместимости с HTML5
How to Export H 264 Video in After Effects CC 2018
You would select Quicktime, then in the Quicktime settings choose the codec, etc. I would recommend downloading and installing the x264 codecs for Quicktime Pro for decent quality h264 renders.
How to Render MP4 in After Effects here:
https://youtu.be/amLBgEqXyB4
Download QuickTime here:
https://goo.gl/eE5XVQ
H.264 files should be in an MP4 container. Apple never did a good job supporting h.264 in a QT container, it was buggy and unreliable, and they are not doing any further development. That is why it was removed. It should have been removed a long time ago.
Note: BTW, this question has been asked and answered hundreds of times. Many of those answers are on many forum and they are all the same. Do not use H.264 Quicktime. It's a dead format and h.264 is no longer available in the latest version of After Effects.
But you can export as MP4 format.
► Render as MP4 here:
https://youtu.be/amLBgEqXyB4
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