Use FFMPEG (free software) to easily convert .mkv to .mp4
.mkv files may have poor performance in some video editing software despite having the required codec support. By using FFMPEG the audio and video stream can be remuxed into a .mp4 file with no loss in audio or video quality.
Link to FFMPEG: https://ffmpeg.org/download.html
An excellent video on how to install and set it up: • Install FFmpeg on Windows 10 - 2019
Command used:
ffmpeg -i "invideo.mkv" -c:a copy -c:v copy "outvideo.mp4"
Change the "invideo.mkv" to the name of your input file
Change the "outvideo.mp4" to the name of your output file