When I try to convert a .mkv file, I get the follow error. "Unsupported Video Codec: MPEG/ISO/HEVC". Any clue on what's going on?
That message simply means that you are lacking the necessary encoder to convert the MKV file. Try installing the CCCP Codecs and then repeat the procedure. If you still have issues after this operation, access the website of the developers that released the application you use to convert the MKV files and report this issue.
Download CCCP Codecs: http://www.cccp-project.net/