>>942925 (OP)
Because that tool is shit. Download Open shot, it’s a free video editor that’s easy to use. Download https://sourceforge.net/projects/xdman/?source=directory it’s a video downloader, it’ll let you pull video from most sites. Once downloaded, import the video into your video editor, edit it, save as MP4 (@ correct resolution). Open file in your video downloader, convert to WebM.
Or maybe openshot has an extension that allows you to render videos as webM’s, I haven’t looked.