Davinci Free under Linux doesn't support H264. To edit videos, you need to convert them. Example (from this reddit)
ffmpeg -i input.mp4 -c:v dnxhd -profile:v dnxhr_hq -pix_fmt yuv422p -c:a pcm_s16le -f mov output.movTimeline scrubbing performance is way better with this codec, but comes at the expense of much greater storage. So likely you'll want to only convert those you need for a project, and delete them when done. Or else buy a ton of storage.