If you are trying to pull a quick draft out of Adobe After Effects, remember that older versions do not support direct MP4 exports from the native render queue; you must send the composition directly to Adobe Media Encoder and select the H.264 format preset to produce a functional MP4 file.
Applications like VLC Media Player are highly recommended because they can bypass minor file corruption and read a massive array of missing codecs.
Ensure your system reads it as a standard video container.
AE 23.0.0 - can't import a certain MP4 file - Adobe Community