mp4 video exported from x265 does not play
It looks like the x265 codec needs to be installed for the App you are using to play the file with.

VLC does support x265 with a dd on codec but it seems the link on there site is broken
https://www.videolan.org/developers/x265.html which is not very useful right now.

there are some other players out there doing a quick Google brought up some results but I dont have a x265 file to play with to test them.

if you exported the file, have you tried exporting it again to see if the same issue happens ?