@Jaime-Guaman MKV is just a "container" for the actual video data, which will be in a particular format eg H264, HUFF. It will probably be that format that VPX can't open without the codec being installed.

Install Mediainfo and open your MKV in it. On the View menu, choose Text view, and copy and paste all that info to a post here. Then we'll see the details of the video format and go from there. It could be a simple case of installing that codec and you'll be OK.

If that doesn't work, you may have to convert the MKV into another format which can be done without losing quality.

If you can, could you post a small sample MKV here for us to analyse?

Thank you very much for your quick response, here is the information from the file:

General
Unique ID                                : 234058534980674299655014030400330838730 (0xB01608BC79126C390C2C02A6F382B2CA)
Complete name                            : Myfile
Format                                   : Matroska
Format version                           : Version 4
File size                                : 19.9 MiB
Duration                                 : 3 min 59 s
Overall bit rate                         : 698 kb/s
Writing application                      : Lavf58.29.100
Writing library                          : Lavf58.29.100
ErrorDetectionType                       : Per level 1

Video
ID                                       : 1
Format                                   : VP9
Codec ID                                 : V_VP9
Duration                                 : 3 min 59 s
Width                                    : 640 pixels
Height                                   : 480 pixels
Display aspect ratio                     : 4:3
Frame rate mode                          : Constant
Frame rate                               : 29.970 (30000/1001) FPS
Default                                  : Yes
Forced                                   : No
Color range                              : Limited

Audio
ID                                       : 2
Format                                   : Opus
Codec ID                                 : A_OPUS
Duration                                 : 3 min 59 s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Bit depth                                : 32 bits
Compression mode                         : Lossy
Default                                  : Yes
Forced                                   : No

@Jaime-Guaman

Thanks Jamie, the video codec is VP9. Are you are to upload the file to the forum (use the blue button with the up arrow) or send it to say google Drive and post the link (or PM it to me?). There are a couple of programs that should be able to convert it but I don't want to lead you up the garden path unnecessarily.

@Jaime-Guaman I've had a play with a similar file (from Youtube). Unfortunately, it looks like you'll have to transcode it into MP4 so VPX can open it (MEP won't open it either).

I'd suggest using Handbrake. You can choose to install it or, if this will be a one-off, you can download the portable version, unzip it and run it, then delete the folder afterwards.

Open Handbrake, then open your file in it. Then set it up as follows:

Then hit Encode and Handbrake will produce a M4V file (same as an MP4) which you will be able to import into VPX. Good luck!

Thank very much, that definitely help me to get to the next level.