Runner / Fixes

Premiere will not
import WebM.

You dragged in a .webm and Premiere either ignored it, greyed it out, or imported audio with no picture. WebM is not a Premiere format and never has been.

The single command

ffmpeg -i input.webm -c:v libx264 -crf 18 -preset slow \
       -pix_fmt yuv420p -c:a aac -b:a 192k output.mp4

That converts both the video and the audio, which matters because WebM audio is usually Opus and Premiere cannot read that either.

Converting a folder of them

for %f in (*.webm) do ffmpeg -i "%f" -c:v libx264 -crf 18 ^
    -pix_fmt yuv420p -c:a aac "converted_%~nf.mp4"

That is Windows cmd. In bash it is for f in *.webm; do ... done.

Why renaming does not work

Renaming clip.webm to clip.mp4 is the first thing everyone tries and it never works. The extension is a label on the outside of the box. The VP9 or AV1 video is still inside it, and that is what Premiere cannot read. You may get the file to appear in the import dialog, and then it fails or imports with no picture.

If it still fails after converting

• Check the output actually re-encoded. If you used -c:v copy, the original video stream is still in there.
• Check -pix_fmt yuv420p was applied. Some sources are 10 bit or 4:2:0 variants that Premiere is fussy about.
• Run ffprobe on the output and confirm it says h264 and aac.

ffprobe -v error -show_entries stream=codec_type,codec_name -of csv=p=0 "output.mp4"

Related: the vp09 error, clips with no sound.