Webm is not a supported container.
There are 2 ways to get to a supported container: 1) Change container and 2) recompress.
Since webm supports VP8, VP9 and AV1 and Resolve only supports VP9 (
https://documents.blackmagicdesign.com/ ... c_List.pdf ) you need to make sure it's a VP9 file before progressing the first path.
If it IS a VP9 it's a simple container change. Any ffmpeg frontend (or the program itself) can do that.
The second path is to convert the codec to a supported one. There are many options here, for true losless/ease of use some people prefer DNxHR/Prores, I prefer mpeg4 (the fmpeg codec, not the container) Constant Quantizer 0.
I prefer Xmedia Recode (
https://www.xmedia-recode.de/en/download.html) but handbrake or Convert V6 are other options. The workflow is the same (as they are all ffmpeg frontends): Open program, open files, set container (MOV or MP4), set video codec (as above), set audio codec, convert.
I have tested that despite what that Supported Codec List says, Resolve supports VP9 in a MP4 container.