If the link to the webm file is red - then something is wrong other than ffmpeg not being installed (thats probably a problem too, but the lunk should be blue regardless)
If you want to install ffmpeg from git, that probably means compiling from source. This can be a bit of an involved process if you are not familar with linux commandline. You would need a c compiler (gcc or clang) installed on the server for this to work,many web servers do not have this installed. Otherwise follow the instructions at https://trac.ffmpeg.org/wiki/CompilationGuide/Generic . In a situation where you dont have root (which is your situation) you should ensure that you pass a --prefix option to ./configure that is somewhere in your home directory.
If possible i would reccomend instead to try and convince your host to install ffmpeg via the system package manager. Failing that, for people not familar with linux commandline i would reccomend trying to find some precompiled (preferably static) binaries you could transfer to the server instead of compiling yourself. E.g. like https://johnvansickle.com/ffmpeg/ - you have to pick your platform. Amd64 is the most likely answer by a wide margin, but you can check using the uname -a command.
Hope that helps