You'll need to open that MP4 and see what's inside (There are many different kinds of MP4s). Download the free program Media Info and open your MP4 in there. Copy or post Media Info's report here and we'll better be able to diagnose your issues.
BTW, you DO have Quicktime 7 installed on your system, per PRemiere Elements' system requirements, right?
Thank you for your reply.
Hmm. I do not have QT7 installed b/c I thought it was not necessary with windows 10. Is that wrong?
Quicktime is listed as required in the Premier Elements 14 and earlier specifications. For a variety of reasons, it was eliminated as a requirement in version 15. Still, there may be certain video formats that need the support files of Quicktime. If you watch the installation process carefully, it will give you an option to install only the support files.
Windows versions have nothing to do with this.
Thank you. That did it.