MKV is a container format which is not directly supported by the software. What makes container formats (such as AVI) special is that they can contain a number of different codecs combinations.
If you're having trouble importing an MKV file, you can try "demuxing" it. Demuxing an MKV file separates the audio track from the video track and extracts them from the MKV container.
At the same time, an MPV file (video) and an MPA file (audio) are generated, which you can import into your project timeline.
If the program won't import these files either, we recommend converting the MKV file to MPEG2 or MPEG4.
For more information, refer to the following articles: