Make sure you have the latest FFmpeg and libavcodec packages installed as this might not be the case by default in some Linux distributions.

Please elaborate and guide through the installation process. Thankx Btw

Well it may be in your package manager or apt or yum or whatever you have for your Linux distro.

Your useragent Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:77.0) Gecko/20100101 Firefox/77.0

It appears you are using a flavour of Ubuntu? so perhaps see https://linuxconfig.org/ubuntu-20-04-ffmpeg-installation or https://help.ubuntu.com/community/RestrictedFormats

Patent and copyright restrictions are among reasons why many Linux distros do not come with packages such as FFmpeg in a new install.