I am a bit green when it comes to editing. I have a nextbase camera that records at twice the speed. It is MP4 video. I try and import the videos and I get "importer reports generic error". They are stored on external disk. I have downloaded some but when I edit the video then it asks me where the video is. Very irritating and getting sick of this program as it continually crashes.
I looked up 'nextbase'. It is a dash camera. In video, "MP4" is one of several "container" files that are full of the pieces and parts that make up a video. One is the video 'codec' and there are many. Premiere Elements is intended to edit the codecs used in common consumer video cameras. Dash cams, screen capture software, some phones and some action cameras use unique codecs or formats that Premiere Elements will balk at. Even though playable, footage is not necessarily editable.
The short term solution is to pre-process you dash cam footage with the (free) program called Handbrake. It will convert your files to a more common and editable format.
More time consuming would be to examine the structure of you dash cam files with the (free) program called Handbrake. It might be possible to identify what is out of the ordinary.