Premiere Elements 14 on Windows 10 (assumed 64 bit).
Please give the properties of these .mp4 files that will not play in Premiere Elements. Telling us the brand/model/settings of the camcorder that recorded the video may give us the troubleshooting information needed.
The latest version of QuickTime 7 is 7.7.8 and this has problems updating to. Please install QuickTime 7.7.6 on your Windows 10 computer.
It is not the officially the latest version of QuickTime 7, but it meets the Premiere Elements requirement for QuickTime 7. Do not bother with 7.7.7 if that is what you are using now.
Let us start here and then decide what next based on the details in your reply.