@minajahan
Create a folder lib_extra in /usr/lib/x86_64-linux-gnu/opera/.
Download from https://repo.herecura.eu/herecura/x86_64/ the file opera-developer-ffmpeg-codecs-114.0.5735.9-1-x86_64.pkg.tar.zst (direct link).
Extract libffmpeg.so from the archive and copy it to /usr/lib/x86_64-linux-gnu/opera/lib_extra/
Restart Opera.