Plex does not support the AV1 codec. Unknown if/when AV1 will be supported. Plex generally does not release product plans.
You will need to download in another format or convert existing videos to a supported format (H.264 or H.265).
You may wish to vote for the feature suggestion: Add support for av1 coding standard.
Or just drag the file into the message window. It will insert a the cursor location.
