Debian version should not matter, I have both Debian 9 and 10 running and that the game you are running is supported.
CS:GO is supported, just make sure that you downloaded the correct Metamod for Linux and not Windows or Mac.
Go to:
https://www.sourcemm.net/downloads.php?branch=stable
Then follow the guide:
https://wiki.alliedmods.net/Installing_Metamod:Source
Should look like example: /home/steam/csgo/addons/metamod
Make sure you have set the correct permissions for the newly extracted contents, that the owner of the folder is the same and that you have permissions.