How I convert MP4 video to Webm muxer

[abc123]

I'm trying to convert an MP4 video to Webm muxer, for uploading to the Hebrew Wikipedia, and I get this error message:

How I can convert the MP4 to Webm correctly without errors?

[eumagga0x2a]

This is not possible with Avidemux due to VP9 (or VP8 at least) encoder missing, please have a look at FFmpeg. An Avidemux VP9 encoder plugin is on my todo list, but it most likely won't be in any usable shape in time for the next release.