You need to enter this in the terminal. Navigate to your vault (where the .git folder is, don’t go inside) and run it (if that’s what you need to do; I don’t know this command). Git bash on Windows, normal terminal suffices in Linux (you need to install git package).
I recommend downloading and installing GitHub Desktop on your OS and then look out for the Git related prompt.
Git Bash is the command line tool where you will be able to run the command. I don’t know if Obsidian Git can be configured to run specific commands (stopped using it a long time ago).