WhatsApp video doesn't play and has a warning icon (Video format h264 not supported) Using WhatsApp version 2.20.206.24 on a Huawei Y9 2019 with Android 9.
Received the following video (i tested both forwarding and sending the video directly - on both cases i faced the same problem)

If I click play, the video looks like is going to start but the time is stuck

Checking the media associated with that chat and it appears a thumbnail with a warning icon in it

I'm able to play other videos, so it makes me think that the problem isn't with the media player of this phone. Tried also restarting the phone, searched for WhatsApp updates and system updates and all were in place. Also, I tested playing the video with two other different Android phones (one Samsung and another Huawei) and the video played well.

Edit
When trying to play the video from the Gallery, I get the following error

Formato video h264 não suportado. (Portuguese for &quot;Video format h264 not
supported&quot;).



Edit 2
I installed VLC for Android to see if the video would play using it. The timer this time finished playing as well as the audio but the image was stuck in the same moment
Assuming that the person that sent you the video has a Gmail account, ask to upload the video to Google Photos and send the link, that way you'll be able to see the video.